Multi-Stage Delayed Blowback

[Dreamcast270mhz]

This is a new design I have considered, it being an original design based on the increasing popularity of delayed blowback arms. The target platform is portable machine guns and not infantry rifles. The rough diagram illustrates the construction of the bolt and its operation. Anything striped is a spring, the toggle is on the far right upper. Sorry for the poor quality but my scanner is messed up (again!)

The basic operation, after ignition:

1. Front end of bolt recoils into the second piece, which, being more massive slows down the travel of the bolt.
2. both pieces, now locked together, hit a pair of toggle arms (one being illustrated) which contain a fairly stiff spring.
3. The toggle arms bend upwards, like the Luger pistol, and delays extraction of the cartridge until pressures drop to safe levels.

Then the bolt returns to battery, everything returning to initial position.

This system will use the fluted chamber design to aid in extraction, and the bolt itself will be sealed against fouling, the rearmost portion having a sharp edge to scrape fouling from the foremost portion as it recoils. The gun is intended to have its cyclic rate modified by either tuning the spring or changing out the bolt for a heavier or lighter one.

I am drawing up a machine gun design, taking elements of the ZB26/30, M60, Bren, Type 97 Heavy MG and P90, and hopefully you all can give me some good criticism/suggestions.

 

[tyeo098]

1. Front end of bolt recoils into the second piece,

If this breaks the bolt/barrel/chamber seal, you'll have some nasty scars to show for it. As I assume that Newton will have some say in how far and fast the first part will go.
But if it doesnt, and instead, something like the whole system (carrier/bolt) moves backwards, what you have is a complicated roller-esque delay system with lever arms and more mass to slow down bolt travel.

also

The toggle arms bend upwards, like the Luger pistol, and delays extraction of the cartridge until pressures drop to safe levels.

The toggle arm isnt what is delaying extraction in a Luger pistol. What delays is is the whole system recoiling backward while breech is still locked. (like a 1911)
The toggle arm is just what unlocks the breach, extracts, compresses a spring, and the loads and locks the breech again.

The toggle arm on a Luger is like the barrel hinge on a 1911. It swings the barrel down, unlocking it from the slide, and allows the slide to travel back further (without the barrel) to extract and load a new round.

You might be thinking lever-delayed action? Thats totally different.

also,

the rearmost portion having a sharp edge to scrape fouling from the foremost portion as it recoils.

This sounds like it would wear down fast and/or need to be replaced often.


For a portable MG, why not just use a beefed-up long stroke gas system?
If accuracy is your thing, and not reliability, why not use a beefed-up DI system?

 

[Dreamcast270mhz]

No, you misunderstand. I know the luger is a locked breech, I mean the toggle arms bend the same way, in an upside down V shape. I have considered ditching the two piece bolt too....


Also, I want an original design, one that operates differently and hopefully more effectively. The long stroke in MGs is cliched and direct impingement, well lets just put it bluntly: I hate the stoner system.

The intention of this design is not a primary portable, more like a medium machine gun that is used to provide a larger squad some covering fire without replacing the FN minimi as the principle SAW. As you have probably guessed by now, the machine gun is magazine fed, so keep that role in mind.

Scalability is also a factor, in other words i thought about a system where size and weight may change from variant to variant, lets say a 7.62 NATO and a .50 BMG for simplicity. The 50 BMG is obviously going to be 10-20 pounds heavier due to heavier bolt, springs, receiver, toggles etc. But the design is the same, simplifying maintenance and training procedures.

 

[PercyShelley]

What you have drawn appears similar to the schwarzlose machine gun and Pedersen rifle, which both were blowback operated and used a toggle joint to slow opening of the breech. Only difference was that in those designs the bolt was in communication with the toggle at moment of firing. I am not sure what the purpose of the hang phase before the bolt presses against the toggle in your design it.

 

[Shear_stress]

Front end of bolt recoils into the second piece, which, being more massive slows down the travel of the bolt.

The point of having a massive bolt in a blowback system is to slow the opening of the breech. Having the lighter piece of a two piece bolt directly contact the cartridge is a recipe for out of an battery explosion. Also, it's generally not advisable to give your moving parts an opportunity to accelerate before they contact each other.

 

[PercyShelley]

Actually, there have been a handful of designs that allow the bolt and cartridge to retreat as far as a few mm before locking. Look up the Remington 51 and 53, though those used fairly weak cartridges. The SIG MKMS/MKPS used a similar system as well using the fairly strong 9x25mm Mauser export round. According to some patent data this design has been used for full power rifle cartridges as well (either .308 or .30-06, don't recall off the top of my head) on at least an experimental basis.

If you look at the feed ramps in a number of bolt action and automatic pistol designs you'll be surprised at just how much unsupported case head you can get away with. In this design the initial movement is probably far less of a deal breaker than many here think.

Thing is, I'm completely unsure what purpose the initial case head movement serves. There is a definite reason for it in the Remington 51, but I do not see what it achieves here.

 

[Shear_stress]

Actually, there have been a handful of designs that allow the bolt and cartridge to retreat as far as a few mm before locking. Look up the Remington 51 and 53, though those used fairly weak cartridges. The SIG MKMS/MKPS used a similar system as well using the fairly strong 9x25mm Mauser export round. According to some patent data this design has been used for full power rifle cartridges as well (either .308 or .30-06, don't recall off the top of my head) on at least an experimental basis.

This is true as well in the CETME/H&K design, but that initial bolt head movement is very small (it's needed to extend the rollers) and bolt velocity is significantly slowed by mechanical disadvantage as momentum is transferred to the carrier.

Thing is, I'm completely unsure what purpose the initial case head movement serves. There is a definite reason for it in the Remington 51, but I do not see what it achieves here.

Agreed.

 

[Dreamcast270mhz]

The hang phase is/was intended to loosen the case from the case wall. The second part of the bolt is what seals the breech, the forward piece extends into the bottom of the chamber.

But, now I'm just thinking a fluted chamber will suit just fine by itself so the hang phase may/may not be in a later stage of the design.

And Percy is right, the pedersen and Schwarzlose were two designs I considered.

The MG will have mixed mode firing, firing from closed bolt in semi and open in full auto, like the M60. The selector is built into the charging handle, if the bolt is forward it will fire a single shot, if rearwards, full auto. The weapon is striker fired, and open bolt is achieved by releasing the bolt and striker locked together, and the sear holding the bolt back. In closed bolt, the sear holds the striker back while the bolt is forward.

 

[PercyShelley]

This is true as well in the CETME/H&K design, but that initial bolt head movement is very small (it's needed to extend the rollers) and bolt velocity is significantly slowed by mechanical disadvantage as momentum is transferred to the carrier.

I wasn't being entirely clear. Unless I'm missing something, in the Vorgrimler roller design the design is in battery with the cartridge forward all the way in the chamber, the bolt head pressing against the case head, the rollers pressing against the bolt head and sitting inside the trunnion and the bolt carrier pressing against the rollers.

There is initial movement of the bolt head, but it is slowed by a combination of friction within the trunnion and the mechanical ratio made by the angle of the... forgot the name of the angled piece on the bolt carrier that doesn't have an English name.

In the Remington 51, MKMS and the experimental battle rifle alluded to in the patent documents the bolt and cartridge slide freely for maybe a mm or two under the blowback force without any kind of frictional or mechanical ratio whatsoever. Yeah, I'm kinda surprised that they get away with it too, but there you go.


Edit:

http://www.freepatentsonline.com/2774283.html

There's the patent. The research was apparently being done by Earle M Harvey when he was chief of weapons research and development at Springfield Arsenal. His next major design, the T25 rifle used a positive, non-moving locking system somewhat similar to the VZ-58, so I guess he abandoned the design disclosed in the patent for whatever reason.

 

[45_auto]

Looks like you're trying to design a straight blowback operated system using rifle-power level cartridges. Good luck!

If you're mechanically locking the breech by some other method besides bolt mass and spring force, it's not clear in your description.

1) What delays the front piece of your bolt from moving and unlocking the chamber until the pressure has dropped? Answer - it's mass and a spring. EDIT - Just read your post where it doesn't unlock the chamber, just moves within it - that's a VERY hostile environment for moving parts!!

2) Assuming the front piece does move but doesn't open the chamber (we're talking about maybe a couple of thousandths of an inch here) what delays the two locked-together pieces from moving until pressure has dropped? Answer - their mass and a spring.

Your whole system depends on the mass of the bolt parts and springs to delay opening until chamber pressures drop. Historically, that hasn't proved practical in rifle-power (much less 50 BMG) cartridges. You need some kind of delayed blowback system to hold the bolt closed until the chamber pressure drops. You may want to do a little research before investing much time in your current design. At least read the Wiki article on the different methods that have been used to delay the opening of the bolt in blowback operated firearms.

http://en.wikipedia.org/wiki/Blowback_(firearms)

 

[Shear_stress]

There is initial movement of the bolt head, but it is slowed by a combination of friction within the trunnion and the mechanical ratio made by the angle of the... forgot the name of the angled piece on the bolt carrier that doesn't have an English name.

I believe the initial movement of the bolthead is to extend and force the rollers against the barrel extension.

In the Remington 51, MKMS and the experimental battle rifle alluded to in the patent documents the bolt and cartridge slide freely for maybe a mm or two under the blowback force without any kind of frictional or mechanical ratio whatsoever.

That's my understanding as well. As you said, you could get away with it in the Remington at least because it fired a .32 or .380, depending on the model.

 

[PercyShelley]

Straight blowback scales far better than most people give it credit for, provided you make some small alterations to the design of the cartridge case head. Look up how the type-99 wing cannons in the zero-sen fighter worked...

As for the idea of a blowback GPMG/SAW, I'm not convinced it will be a happy marriage. Ever cleaned out a CETME or HK after a few hundred rounds? The things get filthy, probably worse than an AR-15 does. This is a direct result of the fluted chamber.

Blowback designs also tend to be somewhat sensitive to barrel length, especially when you start adding delay mechanisms and whatnot. For a multi-purpose, multi-caliber design this does not strike me as ideal.

 

[PercyShelley]

I believe the initial movement of the bolthead is to force the rollers against the locking piece.

The Vorgrimler design is pretty complicated, and to be honest I've never been completely sure that I understand all the nuances. That said, if there is some initial gap between the rollers/bolt head/locking piece, then what exactly is pushing the bolt head forwards into battery?

That's my understanding as well. As you said, you could get away with it in the Remington at least because it fired a .32 or .380, depending on the model.

My understanding is that Remington 51 only used that system to get around a patent by John Moses Browning on having the breech face directly on the slide of the pistol. It wasn't necessary, strictly speaking, to have any sort of delay system in a .32/.380, but it did save some weight and size. Remington 51s are freaking tiny.

The SIG and Springfield Armory designs are harder to explain away. It would appear that you can get away with at least a few mm of completely undelayed case head and bolt movement using some fairly hefty cartridges. This absolutely blows my mind and seems insane and unsafe, but it works well enough that it doesn't immediately detonate into a billionty tiny pieces.

 

[45_auto]

Straight blowback scales far better than most people give it credit for, provided you make some small alterations to the design of the cartridge case head. Look up how the type-99 wing cannons in the zero-sen fighter worked...

You may want to do a little research on the Type 99. It's an Oerliken based Advanced Primer Ignition system (discusses in the blowback Wiki article I provided the link to a couple of posts above), not straight blowback. The primer is ignited BEFORE the bolt closes, while the bolt is still traveling forward at a high velocity propelled by the giant recoil spring. The chamber pressure has to overcome the momentum of the bolt as well as the mass of the bolt and recoil spring force (look at the HUGE recoil spring around the barrel!). You definitely won't be using it in mixed-mode (open and closed bolt cycling) like the OP discusses in Post #8.

It's amazing how much you can learn and how much time and money you can save if you try to understand WHY things were done a certain way in existing designs!

 

[PercyShelley]

You may want to do a little research on the Type 99. It's an Oerliken based Advanced Primer Ignition system, not straight blowback. The primer is ignited BEFORE the bolt closes, so the pressure has to overcome the momentum of the bolt as well as the mass of the bolt and recoil spring pressure (look at the HUGE recoil spring around the barrel!).

Think about that for just a second and see if it explains everything that you know.

According to Hatcher's Notebook a straight blowback .30-06 would require something like a 20 lb bolt; clearly impractical. 20mm cannon is far more powerful than .30-06, and if the required bolt mass is departing so non-linearly from a one two two pound bolt in an SMG to twenty pounds by the time it's in full power rifle territory, then a 20mm blowback gun is clearly untenable. According to the figures in Chinn, a straight blowback 20mm cannon ends up with something absurd like a 500 lb bolt.

The API trick does help save some bolt mass, but for a bolt velocity v the cartridge is now accelerating the bolt from -v to v, for a total acceleration of 2v instead of going from 0 for a stationary bolt to v. Work is mass times acceleration, so that's double the acceleration which means that bolt mass can be lopped in half. A 250 lb bolt is still untenable.

Furthermore, if the ignition were to occur while the bolt were still moving forwards, wouldn't that necessarily mean that the case was not all the way in the chamber? You could try to time it exactly precisely at the instant the bolt came closed, but that's a rather tricky bit of timing to pull off when the penalty for getting the answer wrong is losing half your effective bolt mass.

The other critical ingredient in a working Oerlikon cannon is the rebated case head:

http://warrelics.eu/forum/military_...erman-ww2-mk108-20x128-british-naval-20x8.jpg

The middle round is from a Mk 108 cannon.

The rebated rim allows the bolt to follow the cartridge into an extra-long firing chamber:

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

if your allowable distance d is much greater before the case head becomes unsupported, the allowable bolt velocities go up and thus the allowable bolt masses go down. This trick is entirely applicable to closed-bolt designs, though to my knowledge nobody has so applied it.

 

[deadin]

I seem to remember reading about an experimental delayed blowback 30/06 that was designed by Col. Thompson (of sub-machinegun fame.) I believe it used a variation of the Blish lock (which was shown not to be very effective with 45 ACP.) Anyway, apparently the ejection was so violent that it would stick cases in a wooden door at the side of the firing line.........

 

[PercyShelley]

Oh yes, the Blish Lock, or as my brother calls it, "wishful thinking delayed blowback."

Here's the relevant passage from Hatcher:

The Thompson autorifle that I tested at Springfield Armory in 1921 worked as long as the oiled pads [to lubricate the cartridges] were used, but it opened with extreme speed and ejected the cartridges with such force that the empty shells in one test were actually stuck into a heavy wooden door about twenty feet to the right of the ejection port.

(Emphasis mine)

And people complain about the lively ejection from mini-14s...

 

[45_auto]

It's pretty easy to ball-park the bolt weights per Hatcher. A 45ACP Hipoint slide weighs about 3 pounds. A 45ACP has about 500 ft-lb of energy. A 30-06 has about 3000 ft-lb, so it's bolt would need to weigh about 7 times as much, or 21 pounds. A 50 BMG has about 12500 ft-lb, so it's bolt would need to weigh about 25 times as much, or 75 pounds (an M2 weighs about 80 pounds total, pretty easy to see why Browning didn't go in that direction ...). I don't know what a 20mm develops offhand, but it would follow the same physics, which would obviously give you the 500 pound number.

Furthermore, if the ignition were to occur while the bolt were still moving forwards, wouldn't that necessarily mean that the case was not all the way in the chamber? You could try to time it exactly precisely at the instant the bolt came closed, but that's a rather tricky bit of timing to pull off when the penalty for getting the answer wrong is losing half your effective bolt mass.

Easiest just to copy the description:

Description

The Oerlikon cannon and its derivatives feature blowback operation: The bolt is not locked to the breech of the gun on the moment of fire, but moves freely to the rear while the propellant gases propel the projectile forward. Advanced primer ignition (API) is used to make sure that the projectile has left the muzzle and the gas pressure in the barrel is down to a safe level before the breech opens, the firing pin strikes the primer while the bolt is still traveling forward so that the gas pressure first has to overcome the forward momentum of the bolt before it can push it to the rear. To give the heavy bolt sufficient forward speed, a large spring is required, which is wrapped around the barrel of the gun. Also, the chamber is longer than needed to contain the case, so that the bolt and case must travel a small distance to the rear before the case extends beyond the face of the chamber. Nevertheless a fairly heavy bolt must be used, which limits the rate of fire.

This design resulted in the use of a characteristically shaped cartridge: The case has straight sides, very little neck, and a rebated rim. The straight sides allows the case to slide back and forward in the cylindrical chamber. The neck is not supported while this happens and therefore expands when the case is fired, and the rebated rim allows the face of the bolt, with its extractor claw hooked over the rim, to fit within the chamber. To ease the motion of the case, the ammunition needed to be greased, which was a drawback of the Oerlikon cannon. An alternative developed during World War II was the so-called fluted chamber, which had grooves that allowed gun gas to seep between the chamber wall and the case, taking over the role of the grease.

Hopefully the OP will realize that we've left simple blowback like he was pursuing a LONG time ago (Blish lock, etc)!

 

[Shear_stress]

The Vorgrimler design is pretty complicated, and to be honest I've never been completely sure that I understand all the nuances. That said, if there is some initial gap between the rollers/bolt head/locking piece, then what exactly is pushing the bolt head forwards into battery?

It really is pretty complicated, and my understanding is far from complete. I believe that as the roller, bolt head, locking piece, kitchen sink and carrier return to battery, they are pushed together and reversibly locked by the locking lever. However, there is a gap between the bolt head and carrier (the "bolt gap") that, on the firing of the cartridge, allows the bolt head to travel backward a small distance so as to force the rollers into the barrel extension. The "roller delaying" part of the cycle then happens

 

[45_auto]

Work is mass times acceleration

As a sidenote, you're incorrect there.

Work is NOT mass times acceleration.

Force = mass times acceleration

Work = force times distance

So Work = mass times acceleration times distance

In the Oerlikon, it looks like the "distance" part of the work equation is where the huge spring force comes into play. Give me a big enough spring, and you can make your bolt as light as you desire! However, if you're looking for automatic operation, you're going to have to balance the two so the cartridge recoil force can cycle the bolt far enough to pick up a new cartridge. It would seem that the long recoil spring wrapped around the barrel equals high bolt closing velocity, which adds cheap energy to the closing bolt that the cartridge must overcome to open the bolt. Since energy is mass times velocity SQUARED, it's more efficient to add velocity rather than mass to the bolt. But this means the bolt has to be actually moving in the closing direction when the cartridge fires, and the cartridge has to be fully supported when it fires, making ignition timing critical. Probably the reason it's not a real common system.

 

[Fish Miner]

Not to be wet blanket on the fire of innovation- but I work in the patent world and anytime I see people disclose inventions on public forums without having proper protections in place it makes my stomach knot up.

The unfortunate reality of this modern world is that intellectual property is stolen all the time and the inventor is usually the one who gets hosed.

please visit: www.uspto.gov


Good luck

 

[PercyShelley]

As a sidenote, you're incorrect there.

Work is NOT mass times acceleration.

Force = mass times acceleration

Work = force times distance

So Work = mass times acceleration times distance

In the Oerlikon, it looks like the "distance" part of the work equation is where the huge spring force comes into play.

D'oh! Yes, thank you for catching that.

Confusing work and force... early onset Alzheimer's.

Anyway, the bolt on a 20mm Oerlikon is in the neighborhood of 30 lbs. As you say, enormous spring. I recall seeing pictures of ground crew with prybars to cock those things.

According to Chinn, it's not so much the total cartridge energy that determines bolt mass in a blowback system, but a fairly complicated function of the peak pressure level, length of the pressure curve, and maximum allowable exposed case head at a given pressure level. Something tells me a lot of this stuff was figured out empirically...

ideally on the other side of a blast-proof window with a long string.

 

[45_auto]

According to Chinn, it's not so much the total cartridge energy that determines bolt mass in a blowback system, but a fairly complicated function of the peak pressure level, length of the pressure curve, and maximum allowable exposed case head at a given pressure level. Something tells me a lot of this stuff was figured out empirically...

Definitely so!!!! I was trying to simplify things to an "internet discussion" level. I've been a mechanical design engineer designing rocket mechanisms for over 25 years now, but I have no practical experience with high speed, high pressure firing mechanisms. It would take me a LOT of research before i even began to try to design a new automatic firearm.

I would imagine there's a gigantic bin filled with blown up barrels, receivers, bolts, etc somewhere in Oerlikon's past! Empirical testing and validation of mathematical models is a HUGE part of mechanical design. I hope anyone else experimenting with the forces involved is well aware of past solutions to help educate them on problems they will be facing.

 

[Dreamcast270mhz]

Let me make myself clear: This is Delayed blowback, in which there is the bolt mass, spring pressure, toggle system (2 of them) not to mention the toggle springs, which can be much stiffer than the main return spring.

The first target would be to get it running under a lower pressure intermediate, such as 6.5 Swiss, Arisaka, or 6.8 Rem.

The gun is not belt fed but magazine fed, and no, I'm not going after the 50 BMG, it was just an example. The largest practical cartridge would probably be .410 in size, any larger and the gun would be too heavy and impractical.

And no I did not consult the Blish Lock, that is an awfully unnecessary design.

I would appreciate a mod to clean up the side talk in this thread too, please.

 

[Andrew Wyatt]

http://www.3ds.com/products/draftsight/free-cad-software/