Why not a blowback .223 semi-auto rifle?

[D Anderson]

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.

 

[holdencm9]

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.

 

[briansmithwins]

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

 

[D Anderson]

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.

 

[Certaindeaf]

It's probably because nobody has ever considered it.

 

[D Anderson]

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.

 

[Ar180shooter]

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.

 

[D Anderson]

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.

 

[Ar180shooter]

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).

 

[Certaindeaf]

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

 

[holdencm9]

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.

 

[D Anderson]

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.

 

[briansmithwins]

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

 

[holdencm9]

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!

 

[D Anderson]

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.

 

[holdencm9]

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!

 

[JohnKSa]

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.

 

[Kurt_D]

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.

 

[briansmithwins]

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

 

[D Anderson]

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

 

[tyeo098]

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.

 

[D Anderson]

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.

 

[Panzercat]

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.

 

[holdencm9]

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.

 

[JohnKSa]

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.