Why not a blowback .223 semi-auto rifle?

Handguns seem to work just fine on the blowback principle.

Why not a blowback .223?

The same principle applies with rifles as with pistols. Blowback works well for lighter calibers but it becomes impractical with heavier calibers. The bolt weight and recoil spring tension have to rise to impractical values once the caliber goes past a certain power level.

handguns operate at MUCH lower pressures than rifles do....

the blowback pistols use very heavy slides to tame the forces (look at Hipoint).....to do that with a rifle, youd need a ridiculously heavy bolt.....its physically possible, just not practical

So what keeps the bolt from blowing back on an AR-15?

So what keeps the bolt from blowing back on an AR-15?

The locking lugs meshed into the barrel extension

So what keeps the bolt from blowing back on an AR-15?
The locking lugs on the bolt.

When a round is chambered, the bolt is locked to the barrel extension and force on the breechface can't unlock it because the bolt lugs have rotated to engage matching lugs on the barrel extension.

The gas pressure routed back to the bolt pushes the bolt carrier back which causes the bolt to rotate and the lugs to disengage from the barrel extension.

The bolt is blown back, but by gas pressure bled from the barrel routed through the gas tube, not by force against the breechface as would occur in a blowback design.

The critical difference is that the breech is locked closed and can't be opened by force against the breechface. In a blowback, the only thing holding the action closed is the inertia of the bolt/slide and the force of the recoil spring.

So what keeps the bolt from blowing back on an AR-15?


the bolt in an ar15 isnt actuated by the blowback from the cartridge, its cycled by the gas that is siphoned off from the barrel, which uses much less force....

Edited, seems to be well covered

When a round is chambered, the bolt is locked to the barrel extension and force on the breechface can't unlock it because the bolt lugs have rotated to engage matching lugs on the barrel extension.

The gas pressure routed back to the bolt pushes the bolt carrier back which causes the bolt to rotate and the lugs to disengage from the barrel extension.

The bolt is blown back, but by gas pressure bled from the barrel routed through the gas tube, not by force against the breechface as would occur in a blowback design.

The critical difference is that the breech is locked closed and can't be opened by force against the breechface. In a blowback, the only thing holding the action closed is the inertia of the bolt/slide and the force of the recoil spring.

Makes sense.

Well a .223 blowback gun is not necessarily impractical. Look at the HK33 or even the G3 which fires the .308. Those are blowback operated guns.

Granted it's a roller-delayed blowback and not just a straight blowback design like your typical SMG. But blowback none the less. I hear they tend to beat the hell out of your brass because of the pretty violent ejection unlike the softer gas operated designs. But for a military rifle that's hardly important. Not like you have soldiers scouring the field after battle to pick up the brass.

Hk43, hk93.

Handguns seem to work just fine on the blowback principle.

Why not a blowback .223?

The Astra 400 is a blow back design pistol chambered for 9mm Largo. About the same power as 9x19.

The recoil spring is very stiff and it is a bear to manually cycle the slide. If you ever see one, try it and you will understand why only low powered pistols are true blow back designs.

One of the aims of a locked breech design firearm, whether the toggle link M1911 system, rotating bolt like in a AR-15, or the various of other designs, is to reduce the pressure in the barrel to a manageable level before the breech opens.

Why not a blowback .223? Because it would require a massive bolt to provide inertia and an extremely heavy spring operating over a long stroke to counter the pressures. You would probably need a winch and pulley to get the action open. That type of action is also high-impact and is absolutely hell on the hardware when scaled above handgun rounds. Metal peens, springs wear out quickly, various parts wind up with reduced lifespans, etc. You name it, all sorts of things go wrong when you try to scale blowback up beyond handgun levels. It's just a Bad Idea and a locking bolt with gas system is the way to go.

Most centerfire handguns are recoil operated rather than blow back. It may seem like splitting hairs, but the main difference is that a recoil operated firearm fires from a locked breech and stays locked until pressures drops. They operate on the rearward force applied to the whole gun (recoil) rather than the rearward force applied to the case (essentially acting as a piston to the bolt).

68wj, recoil-operated is just a fancy way of saying "delayed blowback". It's still a blowback, just one with a semi-locking breech to reduce the slide impulse.

I wonder how strong the action spring would have to be for a blowback .223, that would be a nightmare to cock if someone was ever crazy enough to make one.

Well a .223 blowback gun is not necessarily impractical. Look at the HK33 or even the G3 which fires the .308. Those are blowback operated guns.

Granted it's a roller-delayed blowback and not just a straight blowback design like your typical SMG. But blowback none the less. I hear they tend to beat the hell out of your brass because of the pretty violent ejection unlike the softer gas operated designs. But for a military rifle that's hardly important. Not like you have soldiers scouring the field after battle to pick up the brass.

While the "roleer delayed" part seems like minute detail, it accomplishes the same function; slowing down the opening of the bolt until chamber pressure is at a safe level. No way a straight blowback would be safe in 5.56NATO/.223REM.

The beating of brass seems to be more a function of cycling without any sort of brass deflector. Really, the worst case scenario is the case neck is dented. Many people run a port buffer on HK rifles so that the brass will be in better shape, but also so that it will not go 30 feet away and chip the finish on a rifle valued at $2k+.

You're absolutely right in that retrieving and reloading brass is not a concern in a military pattern rifle like the HK family.

It is entirely feasible to make a blowback 223 but the drawbacks are a huge breechblock, tremendous change in center of gravity as the breech block moves, and a slow rate of fire. Probably some more drawbacks all having to do with a massive breech block slamming into things.

One of the major issues with high pressure blowback mechanisms deal with case friction. If the case is not lubricated it is going to seize in the chamber and gum up the mechanism.

http://img.photobucket.com/albums/v479/SlamFire/Reloading/Case%20Lubrication/Blowbackandlubrication.jpg

There were a number of highpower delayed blowback mechanism used before WWII and they all used oilers. These were all very messy and the memory of these machine guns is virtually forgotten and due to the Army tin can ammunition coverup of 1921, found in Hatcher's Notebook, you can get into unfortunate debates with people who deign their existence and their function. What replaced oilers was the roller bolt and the fluted chamber. The flutes use gas pressure to float the upper 2/3’rds of the case off the chamber walls. The Mauser Device 06H roller breech was not working until the Germans copied the fluted chamber from a captured Russian machine gun. http://en.wikipedia.org/wiki/StG_45(M) The rest is history.

http://img.photobucket.com/albums/v479/SlamFire/Rifles%20various/PTR%2091/HK91boltlocking.jpg


http://img.photobucket.com/albums/v479/SlamFire/Reloading/Case%20Lubrication/ChamberFlutesMP5.jpg

http://img.photobucket.com/albums/v479/SlamFire/Reloading/Case%20Lubrication/FlutedChamber.jpg

You can still see lubrication on blow back cartridges, the most common lubricated cartridge is the low pressure 22 LR. Cases are totally coated in a wax and this helps extraction in blowback mechanisms. In cold weather though, the wax will condense and gum up extraction, something I have experienced a number of times in very cold weather.

68wj, recoil-operated is just a fancy way of saying "delayed blowback". It's still a blowback, just one with a semi-locking breech to reduce the slide impulse.
Utilizing recoil force to unlock and cycle the action is not the same as a shell case pushing back into the bolt. They are 2 distinct operating systems. Or maybe blow back is a fancy term for non-locking recoil operation.

if i understand the workings of the HK roller-delayed blowback system correctly, I guess you can say it's a hybrid of a recoil and blowback operated system.

The recoil from the gun moves the bolt-carrier backward, unlocking the rollers. Once the rollers are no longer locking the bolt tot he breech, then the cartridge pushes back the bolt like in a regular blowback gun.

The one advantage of a recoil or blowback operated rifle is that without the gas tube and piston associated with your typical automatic action, you can have an automatic rifle with an actual free-floated barrel. I believe the HK PSG1 rifle is an example of that.

Well a .223 blowback gun is not necessarily impractical. Look at the HK33 or even the G3 which fires the .308. Those are blowback operated guns.

Granted it's a roller-delayed blowback and not just a straight blowback design like your typical SMG. But blowback none the less.The mechanisms and operation is completely different between blowback and delayed blowback. That's why we differentiate between the two by adding the adjective "delayed" to the latter.

If you think that adding an adjective to a word makes no difference, then I'd be interested to see you post some 25 yard accuracy comparison results for the guns commonly available from Home Depot. Granted, they're caulking guns and staple guns, but guns nonetheless. ;)

I think it was safe to assume the OP was asking about "true" blowback-operated guns, and not any of the menagerie of "______-blowback-operated" guns that may exist chambered in .223.

The recoil from the gun moves the bolt-carrier backward, unlocking the rollers. Once the rollers are no longer locking the bolt tot he breech, then the cartridge pushes back the bolt like in a regular blowback gun.

Not at all, the HK91 system is a delayed blowback operated weapon. The chamber pressure overcomes the resistance of the locking mechanism in the bolt and drives the bolt assembly out of battery. Enough momentum is imparted to the bolt assembly during the blowback to drive the system through a complete cycle.

After the cartridge primer is struck, the pressure in the chamber rises sharply which pushes the cartridge case back against the face of the bolt. When the force on the bolt is great enough, the locking mechanism releases and the gas pressure accelerates the bolt assembly out of battery. The projectile is out of the barrel by the time the bolt actually unlocks because of the inertia of the bolt assembly and the resistance of the locking mechanism. The bolt assembly reaches a velocity of about 25 feet per second as it is blown out of battery.

68wj, recoil-operated is just a fancy way of saying "delayed blowback". It's still a blowback, just one with a semi-locking breech to reduce the slide impulse.

Erm, kind of. I guess you could say that all systems that do not use gas pressure from a barrel port are blowback, in some sense.

Generally speaking - if the barrel is fixed relative to the frame the action is blowback, if the barrel moves relative to the frame during the action cycle, it is recoil operated.

I think that a simple blowback .223 is more feasible than most of the posters here believe. The bolt would not need to be all that heavy. Bear in mind that one of the biggest strengths of the .223 is low recoil. Using a recoil calculator, I found that to achieve the same bolt velocity as a blowback 9mm carbine, a .223 carbine would need a bolt only around 50% heavier. Blowback 9mm carbines typically have bolts around 1.2 to 1.5 lbs. So a .223 would only need a bolt around 1.8 to 2.25 lbs. This is only about one pound heavier than the BCG in an AR. This would make the weapon about a pound heavier than the AR, which would be a disadvantage, but not so much as to be impractical. If you were willing to accept a bolt velocity of 25 fps (as in the G3), this could be done with a bolt weighing only 1.7 lbs.

The recoil spring weight would depend on the free recoil energy. The fre of the .223 is about twice that of the 9mm. However, due to the length of the cartridge, the spring on the .223 will get compressed about twice as far. So the spring tension on the .223 should actually be about the same as the 9mm.

Fluting the chamber would eliminate the need to oil the cartridge. It would work the same as in the G3. Note that in the G3 the bolt and cartridge do start moving back while the bullet is in the barrel. The rollers may not unlock, but the bolt must move backward to make the rollers unlock. So the bolt and cartridge must move back before the rollers unlock.

I don't mean to insult anybody here. I've been thinking about this idea for years, so I had already researched it before reading this post. If you disagree with me, I would be glad to hear your reasons.

Good points D Anderson.

I do not really have the means or motivation right now to check your numbers, but I have faith they are correct. I never thought that the spring and mass of the bolt would be prohibitive to a .223 blowback-operation anyway, so it makes sense in my head. Definitely a heavier system, but not impossible for a person of regular strength to operate.

I guess my main question now that this thread got restarted is not "why not?" but "why?" What major benefits would there be? I guess you could say it is simpler and therefore cheaper, but I already think the DI guns are pretty simple and with AR's going for $600 that ain't too pricey. DI guns run dirty because the gas gets blown back into the receiver, but I think blowback guns get dirty too, because they cannot form as tight a seal with the chamber, just like people complain about steel-cased ammo, since it won't expand as much as brass to conform to the chamber, they get dirtier, which can in turn cause problems. Not only that, but if there is powder still burning as the casing is on its way out...that's not good! And the expansion of the brass is another question I have, if the bullet travels down a 20" barrel in a blowback gun, how far will the case/bolt have moved? Could that rearward travel cause stretching issues for the case down the road? Also, how does that movement affect accuracy? I think blowback guns are inherently less accurate because of all the slop required in the system.

Maybe it takes the bullet 1/1000th of a second to leave the barrel, from primer strike to clearing the muzzle. Most of the acceleration in the bullet is in the first 5-10" of travel, so most of the bolt acceleration will begin right away too...if it takes .001 sec to exit the barrel, even a 10 fps bolt (120 in/sec) will move an eighth of an inch....that seems like a bad thing in a bottleneck cartridge.

It just seems to me to be a lot of effort for little or no gain.

I think that a simple blowback .223 is more feasible than most of the posters here believe. The bolt would not need to be all that heavy. Bear in mind that one of the biggest strengths of the .223 is low recoil. Using a recoil calculator, I found that to achieve the same bolt velocity as a blowback 9mm carbine, a .223 carbine would need a bolt only around 50% heavier. Blowback 9mm carbines typically have bolts around 1.2 to 1.5 lbs. So a .223 would only need a bolt around 1.8 to 2.25 lbs. This is only about one pound heavier than the BCG in an AR. This would make the weapon about a pound heavier than the AR, which would be a disadvantage, but not so much as to be impractical. If you were willing to accept a bolt velocity of 25 fps (as in the G3), this could be done with a bolt weighing only 1.7 lbs.

Something isn't right with those numbers. 9mmP generates ~400ft lbs of energy and .223 1300ft lbs. That energy has to be (minus friction and air resistance) the same on the bullet and the bolt face of the firing weapon. More than a 50% difference between 400 and 1300.

A blowback 556 rifle can be done, the HK series and FAMAS are two examples. There are some other practical reasons why simple blowback operation doesn't work well in rifle class weapons.

BSW

Thanks for commenting. The OP asked "why not?", not "why?" A lot of people were saying that it was impractical, bordering on impossible. I was responding to that. I don't think this would be the next big thing, so it probably isn't all that worthwhile. But it does have advantages. It would be cheaper and it would be inherently rugged. It would have far fewer failure modes, so it might be more reliable (I said might). But the reason I like the idea is simply because the .223 Rem is probably the most powerful cartridge that is practical for a blowback gun. I think it should be done because it can be.

Who says blowbacks are inaccurate? Blowback .22's are used in precision shooting matches. Many blowback 9mm carbines are noted for having surprisingly good accuracy. It's fairly likely that a blowback .223 would be able to at least equal the accuracy of the average semi-auto carbine, which is good enough.

I don't know if case stretch would be a problem, but the fluted chamber might prevent that. The gas is directed around the edge of the case mouth and into the shoulder space. This should prevent the front of the cartridge from being held forward, stretching the case. However, the flutes might damage the outside of the case directly, which might make reloading a problem.

Being that the bullet leaves the barrel at nearly 3000fps, and does most of its accelerating in the first third of the barrel, I would expect the bullet to transit the barrel in about 1/1600th of a second. I assume a 16" barrel as that would be easier to use for a blowback, and that length seems to be the most popular these days anyway. So if the bolt accelerated to 20 fps, which is a very practical speed, it would be about 1/8 inch back when the bullet exits the muzzle. That's assuming that the average speed of the bolt during acceleration was about 16 fps. The .223 case doesn't taper all that much, and the rear portion of the chamber is not fluted, so with the case extracted only 1/8" I doubt that any burning powder grains would be able to slip out. Some gas might escape though, but probably less than the gas that shoots out of the holes in an AR bolt carrier.

It's probably because nobody has ever considered it.

My post above was for holdencm9. I posted it before I saw briansmithwins's comment. To Brian, yes, I was surprised by the numbers myself. Those were the numbers I got when I plugged the appropriate data into a recoil calculator program. I assumed a 124 gr 9mm fired from a 16" carbine, which gives a higher velocity than a handgun. You can try doing it yourself at

http://www.handloads.com/calc/recoil.asp.

You simply use the box for the "velocity of recoiling firearm" as the bolt. Put in whatever bolt weight you like, and it will tell you what its velocity would be.

Apparently, the muzzle energy doesn't reflect the recoil velocity as much as the momentum does. Momentum increases linearly with velocity, whereas energy increases as a square. So while the .223 has about 3 times the energy, it only has about 50% more momentum.

One thing some of you guys are forgetting about in your calculations is the respective pressures that the 5.56 and 9mm operate at. The point of delaying the opening of the breech is to allow the pressures generated by the ignition of the cartridge to drop off to a safe level. To properly calculate this, you would have to look in to other factors such as bolt thrust.

It is also important to differentiate between simple blowback, and the multitude of delayed blowback operating systems. Delayed blowback systems such as lever delayed (FAMAS) and roller delayed (H&K G3) accomplish the same thing as recoil operated (both long and short) and gas-operated systems, where the respective system delays or prevents opening of the breech and cycling of the action until the gas pressure at the breech has reached a safe level.

Keep in mind, most pistols of 9mm Para or larger use some sort of delayed blowback or recoil operation.Your Glock uses a short recoil operating system, where the barrel is locked in to the slide for a portion of the recoil process. Otherwise, the pistol would require a heavy slide and spring, such as Hi-Point firearms.

There is a reason why we do not see any straight blowback firearms in high pressure rifle cartridges... because smarter people than you or I discovered, well over a century ago, that it was impractical and unsafe to do so.

Actually, I just punched the numbers into a pocket calculator and found that the momentum of a 124 gr bullet at 1300 fps and a 55 gr bullet at 3000 fps is almost the same! I guess the recoil velocity is determined partially by both.

Utilizing recoil force to unlock and cycle the action is not the same as a shell case pushing back into the bolt. They are 2 distinct operating systems. Or maybe blow back is a fancy term for non-locking recoil operation.
There are recoil operated firearms (long and short) and blowback operated firearms (simple and a multitude of delayed blowback operating systems). They should not be confused with each other (recoil vs. blowback) and the different categories of each type (long vs. short recoil and simple vs. delayed blowback).

Personally, I'm really jonesing for a fifteen-twenty pound .223. they'll pay extra!

It was determined early on that there are some delayed-blowback .223 guns in the world, so for the sake of discussion I have only been assuming simple or "pure" blowback.

D anderson,

you are correct the OP was "why not?" and I guess "just 'cuz!" is a valid enough reason as any :)

I do think as AR180shooter mentions, the pressures could be a problem.

Taking a .22LR for example, or any other cartridge commonly using blowback operation, they are usually lower-pressure, and relatively straight-cased. So as the cartridge is sliding out of the chamber, it is still relatively supported all around its circumference. I would be worried about the shoulder of a bottleneck cartridge in what is essentially an unsupported condition.

To respond to your other points, I don't think blowbacks are INaccurate, I just think they are less accurate than a bolt gun, or something like stoner's design where the bolt is locked into the lugs until the bullet is well clear of the muzzle. And I definitely think less gas would end up in the chamber than would end up in the receiver, but perhaps more particulates? And they would end up in a worse spot...the chamber. Even chrome-lined, a chamber needs to be real smooth in a blowback to ensure reliability, and if the powder is still spewing hot gas and powder residue as it is pushing its way out of the chamber, I feel that could hamper things.

All that said, I agree it would be interesting to try! I guess if you were brave enough...

(disclaimer: i would not actually recommend trying this without some sort of remote control to pull the trigger for you, mythbusters-style)

you could plug the gas port and modify the bolt of a cheapo AR. If you just cut off the lugs so the bolt was round, it would essentially be a true blowback. The bolt would always be in its rearward position, in compression, but that would be okay as long as you cut the rectangular head off the cam pin, which would otherwise interfere with the rearward travel of the BCG. Add some mass to the BCG by stuffing lead tape or whatever into the rear portion of it. And get a much stronger buffer spring.

If the gun/casing doesn't explode, you'd have your proof of concept.

There is a reason why we do not see any straight blowback firearms in high pressure rifle cartridges... because smarter people than you or I discovered, well over a century ago, that it was impractical and unsafe to do so.

And at that time it was true. But that was before the invention of the fluted chamber and the .223 Rem. cartridge. It's the combination of these two elements that make it viable. The fluted chamber manages the pressure and the .223 provides low enough recoil.

All self loading firearms are either operated by gas or recoil. Blowback is a type of gas operation where the cartridge is a one shot gas piston that acts directly on the bolt, either with or without a delay to prevent opening while the gas pressure is too high.

Find a .pdf of Col. Chinn's 'The Machinegun'. He details the advantages and disadvantages of the different types of actions.

BTW, there have been 30mm autocannons that worked that function fine and that operate via blowback.

BSW

Personally, I'm really jonesing for a fifteen-twenty pound .223. they'll pay extra!

Probably not. But if hi point or someone came out with a tacticool .223 carbine that utilized simple blowback, came in at 10 lbs, 2 moa accuracy, and a pricetag under $300, I bet it would sell. Which is why I have to imagine there are some major roadblocks to do it, that no one is doing it.

Even if the bolt was super tough to pull back, that's fine. I don't mind putting a little muscle into it. I mean, whoever said that your 90 lb daughter needs to be able to cycle the action with nothing but her pinky? There are lots of guns that are tough to chamber rounds. Heck the Beretta Bobcat has a tilt up barrel because it is so dang tough to do!

I would be worried about the shoulder of a bottleneck cartridge in what is essentially an unsupported condition.

That's the beauty of the fluted chamber. It allows gas to flow around to the front of the shoulder, thereby equalizing the pressure on both sides of the shoulder. Thus, the shoulder doesn't need to be supported.

Even chrome-lined, a chamber needs to be real smooth in a blowback to ensure reliability, and if the powder is still spewing hot gas and powder residue as it is pushing its way out of the chamber, I feel that could hamper things.

The G3 has a fluted chamber and does the same thing, yet it has a good reputation for reliability. Yes, I know it is delayed blowback, but the cartridge still starts to move back out of the chamber while the barrel is under high pressure.

I guess I am having a hard time visualizing the sequence of events as the case starts its rearward travel, and when the gases get from the flutes to the space in front of the shoulder, and if the pressures truly equalize. I think the delayed blowback makes it a much bigger deal because the internal pressure of the case will have a huge spike, and taper off, so is it the gases that are equalizing the pressure or is it just that the pressure has tapered off enough by the time the shoulder actually pulls free from the chamber wall to not matter.

In any case your idea intrigues me. Let me know if you do a prototype!

If you disagree with me, I would be glad to hear your reasons. As mentioned, extraction is well underway before the bullet exits and the pressure has dropped. That's never an ideal situation. There are ways to work around it, but they come with disadvantages, however minor.

I think that the wide range of momentums involved for the .223 will be problematic. The gun would need to work with bullets from the middle 30 grain range up to over 80 grains. If you run the numbers, the most stressing case isn't the 55grain bullets, but rather the heavyweights. The range of momentums (and therefore the bolt velocities) involved varies by over 50% from the bottom to the top based on my quick & dirty survey. I'm not a gun designer, but I get the feeling that trying to get the gun to work with that wide a range of bolt velocities won't be simple.

In the environment where people are pushing for shorter barrels, lighter guns, collapsible stocks, etc. I don't think the extra weight required to make the bolt heavy enough is going to be attractive to most folks.

Blowbacks aren't nearly as forgiving as locked breech guns. If someone hotrods a blowback rifle with an overpressure reload, I suspect that the results would be truly impressive given that there's nothing holding the bolt closed other than inertia and a spring. It's one thing for that to happen with a pistol cartridge in the mid 30K psi range. It's another thing to have it happen with a rifle cartridge in the mid 50K psi range.

9mm and a .223 may have similar recoil forces but the pressure these thing operate at are totally different.

In a straight blow back set-up the bolt begins moving as soon as the round fires, the weight of the bolt and spring pressure keeps the round in the chamber until a safe level of pressure is reached for the spent case to exit.

Now in theory a blowback .223 could use the same spring/bolt setup as a 9mm BUT the .223 runs at a MUCH higher pressure and does so for a much longer time. So what would happen? 1st (assuming brass) the case seals against the chamber wall. 2nd the brass stretches as the case head starts pushing back on the bolt; in 9mm the lower pressure and shorter time at peak pressure is easier on the brass. 3rd one of 2 things happens: the case head seperates and 56,000 psi of hot gas blows up in your face OR the case begins to extract, the brass isn't strong enough to support the pressure with out the chamber walls, the case walls splits and 56,000 psi of hot gas explodes in your face.

What can fix this? Either a heavier spring and/or heavier bolt. How much heavier to keep the round in the chamber until the pressure drops to a safe level for extraction? Remember it's not just shear pressure but the lenght of time the pressure stays too high for the brass to handle alone. I don't know but I'll bet there are some engineers that have figured it out. Enter delayed methods of blowback. Ever field strip a hk91/33 clone and lock the BCG outside the weapon? Ever done that with a mp5 clone? I have and you ain't unlocking the 91 BCG without mechanical help or rotating the bolt head, that's also why the charging handle acts as a lever too; the mp5 BCG you can sqeeze the rollers in your fingures or simply pull to unlock, it's charging handle is straight pull too.

For a real life example of engineering making a bad idea workable, look up the Schwarzlose machinegun. It was a rifle cartridge firing, delayed blowback, heavy MG.

http://en.wikipedia.org/wiki/Schwarzlose_MG_M.07/12

Just because you can do it don't make it a good idea.

BSW

Brian, interesting link. So what was wrong with it? Judging from the Wiki article, it was quite successful.

The FAMAS is a blowback 223.
Its lever delayed using leverage to delay the opening of the bolt, but either way its still blowback using a fixed barrel.

As mentioned, extraction is well underway before the bullet exits and the pressure has dropped. That's never an ideal situation.

It's not ideal, but so what? If it works OK, it doesn't matter.

The gun would need to work with bullets from the middle 30 grain range up to over 80 grains.

Not really. Sure, that would be preferable, but lots of guns won't work properly with absolutely every possible load. As with any semi-auto, you should test several and settle on the ones that work best. If you want to use extra heavy or light bullets, you could easily adjust the gun by changing the buffer weight. Also, people use a wide range of different loads in 9mm carbines (from cheap low powered loads to +P+) and they seem to handle all them pretty well. Blowbacks aren't all that fussy really.

If someone hotrods a blowback rifle with an overpressure reload, I suspect that the results would be truly impressive given that there's nothing holding the bolt closed other than inertia and a spring.

If someone uses an overpressure reload, he's taking a risk no matter what gun he's using. I don't think the risk would be any worse with a blowback. The most likely result would be a ruptured case. It probably wouldn't even damage the gun. Anyway, if someone uses an overpressure load and the gun malfunctions dramatically, should anybody be surprised?

I'm not claiming that a blowback .223 would be the best gun ever, just that it's workable.

Probably not. But if hi point or someone came out with a tacticool .223 carbine that utilized simple blowback, came in at 10 lbs, 2 moa accuracy, and a pricetag under $300, I bet it would sell.

And I'll bet he's right.

Since we're on the topic, If you've ever shot a Hi-Point, then you know the slide is made out of depleted uranium. I think the 50% increase in weight matters less than its actual distribution, which frankly sucks on the c9. It's a mostly plastic pistol with almost every last ounce dedicated to the slide.

All that said, I'm sure if they can make a blowback .45 acp work, they can get a .223 to work. The balance would be all wonky, but that apparently hasn't hurt their sales yet. And those damn things seem to keep going and going and going if the torture tests are any indication.

Since we're on the topic, If you've ever shot a Hi-Point, then you know the slide is made out of depleted uranium.

That made me LOL.

If it works OK, it doesn't matter.If it works well it doesn't matter. If it doesn't work well it matters a lot.Not really. Sure, that would be preferable, but lots of guns won't work properly with absolutely every possible load.When I say it needs to work, that also implies it needs to be safe. If the bolt is travelling at one speed for the lightweight rounds and is going more than 50% faster with a heavyweight bullet, there's the potential for extraction to progress too far before the pressure drops and that would be catastrophic. One would have to design for the worst case scenario (heavy bullets) to insure safety but then the bolt speed may not be fast enough for reliable function with the most commonly available bullet weight--55 grains which is on the light side of the balance.If someone uses an overpressure reload, he's taking a risk no matter what gun he's using.Yes, but in a locked breech gun, there's more of a margin of safety because the bolt is held closed by something more substantial than inertia and a spring. And it's held closed until the timing of the gas system opens it. An overpressure round will travel down the bore faster and open the bolt quicker, but even so, short of locking lug failures, the pressure will probably be safe by the time the bolt opens.

With an overpressure round in a blowback system, not only is there nothing locking the breech, the overpressure round will cause more recoil and will actually open the bolt a lot faster than otherwise, making the situation even worse.Anyway, if someone uses an overpressure load and the gun malfunctions dramatically, should anybody be surprised?I'm not talking about a dramatic malfunction. I'm talking about the bolt coming back fast enough to be dangerous and having the partially extracted case do a grenade impression because it's still under pressure due to the too-fast extraction.I'm not claiming that a blowback .223 would be the best gun ever, just that it's workable. You could make it work, as in you could get it to operate with a restricted range of bullet weights, but you still won't have the same safety margin as you would in a locked breech gun. And to get it to operate safely with all commercial loads, you'd likely have to sacrifice functionality with the lighter bullet weights.

So could you make it functional as in "Hey look, it shoots if I restrict the ammo to only these bullet weights!"? Yeah, I think so. Would it be "workable" as in "reasonably practical". No. Which is a big reason why no one is doing it.

I'll add this to the discussion. A direct blowback intermediate rifle is something that has already been tried, and seemed to work OK in the form of the Volksstrumgewehr lines from the end of WW2. Ian at forgotten weapons had the chance to shoot one (http://www.youtube.com/watch?v=anW7HWFlueg&list=UUrfKGpvbEQXcbe68dzXgJuA&index=35&feature=plcp)and judging by everything he said, it seemed to have worked just as well as the StG44. While it is listed as a "gas delayed blowback" as Ian comments,it is highly questionable if that particular system actually ended up doing anything and by and large the gun functions as a direct blowback rifle.

V, thanks for the link. Very interesting. Now I'm even more convinced that this could work. Imagine how much better this could be done using modern materials.

You could make it work, as in you could get it to operate with a restricted range of bullet weights, but you still won't have the same safety margin as you would in a locked breech gun. And to get it to operate safely with all commercial loads, you'd likely have to sacrifice functionality with the lighter bullet weights.

So could you make it functional as in "Hey look, it shoots if I restrict the ammo to only these bullet weights!"? Yeah, I think so. Would it be "workable" as in "reasonably practical". No.

Actually, it would be fine to sacrifice functionality with lighter bullets. They're typically only used for long range varmint hunting, and this gun wouldn't be a good choice for that anyway. It doesn't really need to handle anything over 70 gr either, as bullets that heavy are rather specialized items. It would probably only really need to work well with bullets in the 55-69 gr range. That's what most people would shoot anyway. I don't know if there would be a safety issue with heavier bullets, but there's no reason to assume it. The difference in bolt velocity between a 55 gr and 77 gr bullet wouldn't be that much.

As a side note, a Hi-Point .40 was successfully rebored and modified to fire 10mm (http://www.youtube.com/watch?v=UToeNXDL9WY)without any issue whatsoever. The design is quite robust contrary to popular belief.

If you get a chance, take a look at the FN 5.7. 5.7 operates at similar peak pressures to .223, the 5.7 is ~50,000psi, the .223 is ~55,000psi. The AR57 upper is a direct blowback rifle system and it beats the heck out of your brass. Shoulders are blown forward significantly, like this:
http://www.thefirearmsforum.com/attachment.php?attachmentid=14569&stc=1&d=1193201371
Case splits are not uncommon. I'm not trying to bash the round or the idea, I love my AR57, but the idea of a .223 blowback scares me. .223 develops much larger velocities by holding that peak pressure longer, which would do even more damage to the brass. Fluted chambers don't make that issue go away either; it takes time for the gas to flow and pressure to equalize, it doesn't happen instantly. For a good read on the subject check out this book: http://www.amazon.com/Ballistics-Theory-Design-Guns-Ammunition/dp/1420066188/ref=sr_1_5?s=books&ie=UTF8&qid=1349309131&sr=1-5&keywords=ballistics. I've been slogging my way through it and it's impressive how large a pressure differential you can have between the gas right behind the bullet and the gas in the case.

As a side note, a Hi-Point .40 was successfully rebored and modified to fire 10mm without any issue whatsoever.

True, but 40 and 10mm are much closer in performance than .223 and 5.7 or .223 and 9mm. The fact that both are straight walled cases also makes them much more compatible with a blowback system.

Jon_Snow: Valid point, but I don't think having a recovered cases that you can reload was/is much a concern of this thought experiment. If the case gets trashed after the first use, its not much of an issue. Also, what about the use of say Russian Steel Case ammo? I doubt the shoulder creep problem or split cases would be as much of an issue, since you're not going to be reloading those cases anyway.

Plus I think its a foregone conclusion that a strait blowback .223 rifle is going to have various shortcomings versus a breach-locking rifle, the more interesting question here is can it be done, and how well will it work.

Thinking of the issue of case support and blow back, I don't see any reason why you couldn't use an overly long fluted chamber with a bolt that telescopes INTO the chamber, so that when the system begins to move back under recoil, the case is still supported by the chamber for a good bit of its travel until pressure drops to a more managable level and the case ejects out the side. To this extent, I also wonder if an extractor is even needed, since the case will be driving the whole system back as soon as it fires and will not be "pulled out" of the chamber. Thus, all that is needed is a fixed ejector to kick the case out the side. If I recall correctly, the HK P7 is well known for being able to function just fine with the extractor completely removed from the system, and it also uses a gas-delayed direct blowback system.

Case splits are not uncommon. I'm not trying to bash the round or the idea, I love my AR57, but the idea of a .223 blowback scares me. .223 develops much larger velocities by holding that peak pressure longer, which would do even more damage to the brass. Fluted chambers don't make that issue go away either; it takes time for the gas to flow and pressure to equalize.Yup. And it's tricky to reload for because the bolt timing and pressure curve have to be worked out carefully to prevent catastrophic incidents. The same problems would be encountered with the .223, but amplified. Not only for the reason you point out but also because there's a much wider range of bullet weights and commercial loadings that need to be accommodated safely.thanks for the link. Very interesting. Now I'm even more convinced that this could work. Imagine how much better this could be done using modern materials.The bolt/slide on that gun reportedly weighs upwards of 6lbs--about twice what should be required based on the method you used for calculating the bolt weight required for a .223 rifle. Even so, the designers felt the need to add a delaying system.

So, given that you're convinced a blowback .223 is a viable design, how do you account for the fact that no one is making or selling such an animal?

Yup. And it's tricky to reload for because the bolt timing and pressure curve have to be worked out carefully to prevent catastrophic incidents. The same problems would be encountered with the .223, but amplified. Not only for the reason you point out but also because there's a much wider range of bullet weights and commercial loadings that need to be accommodated safely.The bolt/slide on that gun reportedly weighs upwards of 6lbs--about twice what should be required based on the method you used for calculating the bolt weight required for a .223 rifle. Even so, the designers felt the need to add a delaying system.

So, given that you're convinced a blowback .223 is a viable design, how do you account for the fact that no one is making or selling such an animal?
For the same reason you didn't see many mass marketed higher caliber blowback pistols prior to hi-point-- It's an inefficient, unwieldy design that manufactures assumed wouldn't have a place in the market with consumers...

...Except when your price point is somewhere between "Dirt" and "Cheap".

Jon Snow, does the AR57 have a fluted chamber?

Re: the need to use a very heavy bolt/spring combo in a delayed blowback high-pressure firearm.

Many SMGs rely on a delayed blowback system without the heavy bolt/spring.
Pulling the trigger on a 9mm Sten, Stirling releases the cocked-back bolt which then picks up a round and slamfires it as the bolt is still closing. The bolt's forward momentum plays an important part in delaying bolt blowback till chamber pressure drops.

Right, in an open-bolt design you can substitute momentum for inertia and spring force by having the cartridge fire before it's actually fully chambered. Works best with relatively low-pressure cartridges for obvious reasons.

From a practical perspective, that's not a viable option for the U.S. market since the BATF frowns on open-bolt semi-auto designs.It's an inefficient, unwieldy design that manufactures assumed wouldn't have a place in the market with consumers...

...Except when your price point is somewhere between "Dirt" and "Cheap".All correct. Except that there are obviously manufacturers who design and sell to that price point--and yet we still don't see them turning out blowback .223 rifles. So that doesn't quite answer the question.

So, given that you're convinced a blowback .223 is a viable design, how do you account for the fact that no one is making or selling such an animal?

Because it would be inferior to the proven designs that are already available. When I said it was workable, I only meant that it could be made to work. I never said that it would be commercially viable.

Because it would be inferior to the proven designs that are already available.The Hi-Point pistols are inferior to proven designs already available and yet they are made and sold in great numbers, apparently to the satisfaction of both the manufacturer and at least a significant percentage of buyers.

In other words, the fact that a design is accepted to be inferior to other available proven designs doesn't keep a gun from being made and sold nor from being commercially viable.

Ok, that aside, how do you explain that the bolt/slide on the rifle in the link that -v- provided is reportedly about twice as heavy as your calculations suggest is required and yet the designers still added a delay mechanism?

Ok, that aside, how do you explain that the bolt/slide on the rifle in the link that -v- provided is reportedly about twice as heavy as your calculations suggest is required and yet the designers still added a delay mechanism?

Reportedly? The only place I could find this weight reported was in a blog where they were just basing the weight on a calculation. They didn't actually weigh it. The guy in the video said that the guess sounded reasonable, but he didn't weigh it either. Their estimate could easily be a pound or two off. The 7.92 x 33 has twice the bullet weight of a typical .223. so it would need a much heavier bolt. Based on plugging the numbers into the recoil calculator, it's should weigh at least 3 lbs. I wouldn't be surprised if it weighed 4 or 5 lbs. The extra weight certainly wouldn't hurt its reliability. Even if it did weigh 6 lbs., this would only suggest a 4 lbs bolt for the .223. Pretty heavy, but still doable.

The delay mechanism would have had little if any affect. The gas ports were near the muzzle, so it wouldn't have done anything at all to prevent the action from starting to open early. All it would do is increase the effective spring pressure later in the cycle. This might help buffer the recoil when using higher pressure loads.

The Hi-Point pistols are inferior to proven designs already available and yet they are made and sold in great numbers, apparently to the satisfaction of both the manufacturer and at least a significant percentage of buyers.

In other words, the fact that a design is accepted to be inferior to other available proven designs doesn't keep a gun from being made and sold nor from being commercially viable.

They took a gamble and it worked out. They did this relatively recently. How come nobody was selling something like the Hi-Point 30 years ago? Because improvements in technology (synthetic frames in particular) made it more practical. Also, a blowback pistol isn't all that unusual. There have been been blowback pistols is slightly milder rounds for ages.

Anyway, the shortcomings of a blowback .223 would probably be worse than a 9mm pistol. I doubt it would sell well.

Perhaps some form of hydraulic/gas buffer could be implemented? like a car shock/buffer

Jon Snow, does the AR57 have a fluted chamber?

No, it doesn't but I remain unconvinced that it would help. I does however have a bolt that weighs about twice what a .223 BCG weighs. I'll weigh both when I get home tonight.

Jon_Snow: Valid point, but I don't think having a recovered cases that you can reload was/is much a concern of this thought experiment. If the case gets trashed after the first use, its not much of an issue. Also, what about the use of say Russian Steel Case ammo? I doubt the shoulder creep problem or split cases would be as much of an issue, since you're not going to be reloading those cases anyway.

I wasn't concerned about being able to reload the cases afterwards, I'm concerned about rupturing them during extraction. Restricting the gun to steel cased ammo only might solve the problem, but I'd still be cautious.

In other words, the fact that a design is accepted to be inferior to other available proven designs doesn't keep a gun from being made and sold nor from being commercially viable.

I'm going to play devil's advocate for a moment. Typically, companies don't make things unless they are commercially viable. That means that they must believe that whatever loss in perfomance they have due to a sub-optimal design must be counter-acted by a matching drop in sale price. The fact that no one has commercially produced a blowback .223 just means that the preformance loss wouldn't match a potential price reduction. In short, even if you can make something work, if no one would buy it then no one will make it.

Nice explanation with some real engineering behind it of some of the disadvantages of a high-pressure blowback here:

http://www.orions-hammer.com/blowback/

He comes up with a bolt weight for a blowback .223 of around 7 pounds to keep extraction reasonable.

Nice explanation with some real engineering behind it of some of the disadvantages of a high-pressure blowback here:

http://www.orions-hammer.com/blowback/

He comes up with a bolt weight for a blowback .223 of around 7 pounds to keep extraction reasonable.

Excellent find, sir!

I knew the answer was in there but hadn't had a chance to look for it yet.

BSW

One glaring problem with the analysis in the posted link:
The basic problem here is that though the pressure pushing the bullet and bolt are equal, the areas are not equal. Cartridges are always at least a little bigger at the back end, and sometimes much bigger. This causes "bolt thrust" issues with the new short fat cartridges like 300 WSM, even at quite reasonable chamber pressures. In fact, unlike Chinn, I'm going to ignore the gas momentum and start out by assuming:
pressure on bolt face = pressure on bullet back

As I said in my earlier post, the pressure on the bolt != pressure accelerating the bullet. The chamber pressure is higher, sometimes significantly. So his theoretical bolt weights are too low.

An other for blowback pieces of evidence. The Thompson Blish lock rifle (http://en.wikipedia.org/wiki/Thompson_Autorifle) which used the Blish lock phenomenon with a rotating non-locked blowback bolt design. Lately it has been noted that dissimilar metals moving against each other under high pressure demonstrate fluid-like behavior, so that accounts for the increased friction vs predicted.

(Link to TFB article about this and the Thompson auto-rifle) (http://www.thefirearmblog.com/blog/2012/09/13/sliding-metals-show-fluid-like-behaviour/)

The rifle worked as basically as a blowback rifle, where as soon as the cartridge fired, it would push the bolt back and the bolt would begin to rotate it. Some of the rotation of the bolt and increased friction from the blish phenomena helped to slow the bolt down and give a greater "holding" power then just a strait blow-back or mass. This was by the way with a .30-06 and 7.62x54r and not a .223, so significantly MORE bolt-thrust then what a .223 would deliver. Also, this rifle did need lubricated cases, but that is not anything that can't be solved with chamber fluting, as they serve the same purpose.

Again, I offer historical evidence for consideration that blowback rifle have been made, and they have worked safely.

And to re-iterate, for this discussion, when we are talking about blowback, we mean a system where the bolt is not locked to the barrel, and requires a separate outside mechanical force to unlock it, be it direct gas impingement, short or long recoil operating rod, or in the case of the G3 a second recoiling mass.

Reportedly? The only place I could find this weight reported was in a blog where they were just basing the weight on a calculation.Poking around, I found two other sources which both estimated the weight as being 6 or 7 lbs.The 7.92 x 33 has twice the bullet weight of a typical .223. so it would need a much heavier bolt.If you calculate the momentum, it's more, but it's not nearly enough to get the bolt weight up into the 6-7 lb range. The velocity is much lower which helps to counterbalance the effect of the heavier bullet.He comes up with a bolt weight for a blowback .223 of around 7 pounds to keep extraction reasonable.Which, combined with the estimates of the German rifle's bolt/slide weight makes it clear why the designers added a delay mechanism in addition to the already significant bolt weight given the additional momentum that the German rifle had to deal with compared to a .223.The rifle worked as basically as a blowback rifle, where as soon as the cartridge fired, it would push the bolt back and the bolt would begin to rotate it. Some of the rotation of the bolt and increased friction from the blish phenomena helped to slow the bolt down and give a greater "holding" power then just a strait blow-back or mass.The rifle worked basically as a blowback rifle with a mechanical delay based on the Blish lock phenomenon. Which is to say it worked as a delayed blowback rifle. It's been pretty well established that delayed blowback can be practically workable in rifles--and there are a number of ways to accomplish that delay including the Blish lock phenomenon.

Okay, I've been studying this bolt weight issue and I've concluded that my previous assumptions were wrong. It probably does need to be around 6 lbs. It might be possible to get away with a bit less, but 6 lbs. is in the right ballpark. I think the 7 lbs estimate in the one blog is overkill though. He based that on an 80 gr bullet at nearly 2900 fps, which is not realistic from a 16" carbine barrel. Also his assumption that the bolt velocity needs to be no more than 12 fps is questionable. Many blowbacks have far high bolt velocity.

A 6 lbs bolt certainly makes this less practical and should discourage manufacturers from pursuing this as a product. However, it is still quite possible to make a carbine with a bolt this heavy and still have an overall weight of less than 10 lbs. You would have to use a lot of polymer parts and a thin barrel profile. So it's still workable. But having a gun with more than half its weight in the bolt would make for really poor handling qualities.

And a big thank you to everyone for the interesting discussion!

There actually have been straight blowback sporting arms firing rather hefty cartridges; the Winchester Self Loading designs from around a century ago:

http://en.wikipedia.org/wiki/Winchester_Model_1910

Cartridge for the largest ones was the .401 WSL, capable of flinging a 200 grain bullet at 2150 FPS. Not shabby.

(What genius came up with the wholly misleading and useless term "inertia delayed blowback?")

higher pressure calibers are not commonly used in blowback weapons,
cause you suck and we hate you!

:evil:

http://en.wikipedia.org/wiki/Heckler_%26_Koch_G3

http://i53.photobucket.com/albums/g69/Pion_Hallux/HK25.jpg

But even that is downright prosaic compared to the late WWII plans for a 15mm roller-retarded-blowback aircraft gun (I'm unsure if any were ever made), or for that matter the Becker/Oerlikon family of autocannons, which included the 20mm type 99 and 30mm mk 108. Becker/Oerlikon are often referred to as advanced primer ignition. However, if you do the math, you quickly come to the conclusion that advanced primer ignition can only effect a 50% reduction in bolt mass, and that in practice this figure is usually much lower than the ideal.

The handy US Army small arms design guide (http://www.forgottenweapons.com/small-arms-design-literature/), helpfully hosted at Forgotten Weapons states that the correction factor for bolt mass on open-bolt fixed firing pin SMGs is .75, indicating that at least in small arms, API is about half as effective as the mechanical ideal.

Cartridge for the largest ones was the .401 WSL, capable of flinging a 200 grain bullet at 2150 FPS. Not shabby.Correct. The bolt had a weighted extension that extended down under the wooden fore-end of the rifle.

And people will still argue that the telescoped bolt on the Sa. 23 was somehow a new development...

And people will still argue that the telescoped bolt on the Sa. 23 was somehow a new development...

Didn't JMB design the first telescoped bolt, although in a pistol, not a SMG?

BSW

Yes, JMB was the first, but I don't think his patent was that much earlier than the patent for the WSL rifles. A decade, tops.

I've gotta be honest, I haven't actually read through the whole thread but in response to the original query:

"Why not a blowback .223 semi-auto rifle?"

Perhaps a better question would be: "WHY?"


For those who can't understand the difference between "blowback" and "delayed blowback", I suggest the following simple experiment.

1- Take an H&K 91/G3/PSG1

2- Remove the bolt/carrier assembly.

3- Replace it with the rollerless bolt/carrier assembly designed to function with the plastic case, short range training rounds. Add weight to bolt, put in heavier springs, whatever makes you happy... EXCEPT an actual locking system.

4- Put it in a gun vise, point it downrange and tie a really long string to the trigger.

5- Chamber an M80 ball round. (There's probably something in the design of the practice bolt to prevent this, but if you really wanna know about this stuff, you can probably figure out a way to bypass it. The path to knowledge isn't always easy...)

6- Get behind brick wall and pull trigger with long string.

7- Observe explosion, gather pieces, cry over destroyed bits of the umpteen thousand dollar rifle.

THAT is the difference between "blowback" and "delayed blowback".

In any case, H&K literature on the PSG1A1 doesn't say anything about being blowback operated, they do however mention the "locked" nature of the action.

Quote from HK website:
"It uses the famous HK delayed roller locked bolt system pioneered on the HK G3"

Link:
http://www.hk-usa.com/military_products/psg1a1_general.asp

After reviewing Col. Chinn's book and some others I think I can summarize:

1) A straight blowback operated .223 would have a very heavy bolt, around 6lbs.
2) There are other problems with trying to use simple blowback at rifle pressures, gas leakage, case rupture, very slow bolt velocity.
3) There have been successful retarded blowback rifle caliber weapons. The Schwarzlose, HK series, and FAMAS are examples of successful use of the principle.
4) The quality of materials required (high stress steels) and tolerances required mean that retarded blowback rifle class weapons aren't significantly cheaper than recoil or gas operation.

BSW