Roller-delayed blowback system?

[Blakenzy]

I have been reading about self-loading rifles recently and this type of action has been consistently popping up, particularly in H&K and CETME rifles.

I always thought that blowback actions could only handle relatively weak cartridges such as .380 or .25 pistol calibers and that for anything more powerfull a system that locks the breach and bolt together was necessary. I realize that the roller system makes all the difference but how so?

I have not been able to find a clear description of what happens inside the receiver; the sequence of events after the trigger is pulled. Can anyone explain or provide a link that does? Diagrams or animations would be most helpfull.

Thank you

 

[Shear_stress]

http://www.hkpro.com/technical.htm

 

[stdlrf11]

Great Link.

 

[Drakejake]

Rollers in the bolt head lock into recesses in the trunnion and push the bolt face up against the barrel. When the round is fired, the bolt head moves back, forcing the rollers against the locking piece. The locking piece in turn pushes against the bolt carrier and overcomes the locking lever (really a spring). Once the rollers have pushed out of the recesses in the trunnion and are inserted back into the bolt head, the whole bolt assembly moves back, ejecting the spent round, cocking the rifle, then moving forward under pressure of the recoil spring to strip a fresh round from the mag, and again locking the bolt into the trunnion. Generally, the mechanical work that must be done to unlock the bolt allows time for the pressure in the barrel to decrease before the bolt assembly is blown back. Hence roller delayed blowback. More detailed and exact descriptions can be found by doing a Google search using the terms in the prior sentence. The great advantages of this system are simplicity of design and low manufacturing cost.

Drakejake

 

[mete]

There is all the difference in the world between plain blowback and delayed blowback. There are various types - momentum block [Rem M51] ,HK P7 [gas delay], Pederson Rifle [ toggle system] etc. The plain blowback system depends on only slide weight and recoil spring to resist recoil.The delayed blowback system has a mechanism to delay blowback until the bullet has left the barrel and pressures have dropped.

 

[Hoploholic]

Who else had fits the first time they had to reassemble an HK bolt and carrier group and reinstall it in the firearm? Now I look back on it fondly as a right of passage...and wonder how I maintained enough composure to not break anything.

 

[Blakenzy]

Thanks for the replys guys. That link is very informative. The video at the end showing the cutaway rifle in action (G3?) is really, really cool to say the least. I never thought that you could fire a firearm that was cut up like that . I wonder where I could get access to more videos of the like of other firearms. You could learn so much on the inner workings of guns if only such material were more readily available.

 

[9mmepiphany]

I never thought that you could fire a firearm that was cut up

why not?

the explosion is contained in the barrel, the recoilspring is behind the breechblock and anchored in the buttstock. the required strenght holding it together is along the folds of the receiver.

the great thing about the HK system is that it self regulates itself to accomadate for different power levels

 

[Braith-Wafer]

The Sturmgewehr 57(AKA SIG 510) series of rifles use the roller dellayed blowback too.

 

[Southern Raider]

I realize that the roller system makes all the difference but how so?

Good answers so far, but in layman's terms, the roller lock creates a mechanical disadvantage for what is happening in the chamber. In other words, while the rollers are engaged, the recoil spring force and the effective mass of the bolt are multiplied by a factor determined by the locking geometry.

 

[Tony Williams]

Some draw a distinction between "delayed blowback" and "retarded blowback".

This is used to distinguish between those actions which are locked at the instant of firing, but are then unlocked (almost always by a gas system) after which blowback does its job (these are known as "delayed blowback"), and those in which the bolt is not rigidly locked to the receiver but has a system which slows down the initial bolt movement (which are known as "retarded blowback").

According to those definitions, the StG 45 / CETME / HK G3 actions are retarded blowbacks, not delayed. Most delayed blowback actions are found in cannon, like the 20mm Hispano of WW2.

 

[BigG]

I agree with Tony Williams; Delayed does not give the same meaning as retarded blowback. I think it may be people trying to be politically correct, as retarded is almost never heard today in association with mental state. They now say autistic or some other euphemism. But retarded is the proper adjective to describe the roller locks. Retarded means impeded. Delayed just means delayed, it does not capture the mechanical disadvantage like retarded does.

 

[Slamfire]

I would like to point out something. You continue to hear people state claim that the cartridge case must rigidly stick to the chamber walls or “pressures go up dangerously”. The design of the HK floats the upper 2/3 rds of the case with gas flutes only the rear of the case provides the gas seal. I cut and pasted from the HK web site the following:

“In addition to this momentum, almost simultaneously a second impact, F5, is created which acts between the shoulder of the chamber and the shoulder of the cartridge case because of the fluting in the chamber.

Cartridge case obturation, the greatest uncertainty factor in every automatic weapon, is reduced so much by the introduction and further development of the fluted chamber and has become uniform for all types of cartridge cases, including lacquered steel cases, that operation is reliable in every situation, even under the most adverse firing conditions.”

If you notice, the designers of the HK are claiming extraction reliability is increased when the breech friction is reduced. In fact, the Germans found out about chamber fluting from a captured Russian anti aircraft gun, and when they copied that feature, their roller block rifle stopped ripping rims and jamming on extraction.

 

[BigG]

I would like to point out something. You continue to hear people state claim that the cartridge case must rigidly stick to the chamber walls or “pressures go up dangerously”.

I never read that before. Where did you come upon it? The grooved chamber supports the case just like a solid one, to my way of thinking. It is not oversized (which must be what the statement you quoted implies), just grooved. The brass does not expand into the grooves. If you can find a case, usually about 50 feet from the gun, you will see black smoke on the outside, where the grooves let the gas blow by.

The difference between the G3 roller locks and the machinegun MG42 roller locks is the MG lets the bbl recoil, allowing bullet to escape before breech opening The G3 does not, therefore something has to give when the breech is opening with higher pressure.

 

[Tony Williams]

In fact, the Germans found out about chamber fluting from a captured Russian anti aircraft gun, and when they copied that feature, their roller block rifle stopped ripping rims and jamming on extraction.

The idea was patented in Italy, sometime around WW1 IIRC, and was used in at least one interwar Italian MG.

When a high-pressure military gun is fired, the pressure tends to "stick" the fired case to the chamber. The problem which afflicts all blowback guns (whether simple, retarded, delayed or API) is that there is no primary extraction - the initial movement of the bolt or breechblock which breaks the case free of the chamber (and which happens as a part of the function of all recoil or gas operated designs).

With blowbacks, the pressure which drives the case out of the chamber may not be sufficient to overcome this "stiction". The traditional solution has been to oil or wax the cartridges before they are chambered, but this brings its own problems. Fluting the chamber is a way of getting around this, as it "floats" the case off the chamber walls.

The risk is not of the extractor ripping off the rim, though. With blowbacks, the extractor plays no part in the case leaving the chamber, that is all down to internal chamber pressure. The extractor is basically there to pull an unfired case out of the chamber.

 

[Southern Raider]

The extractor is basically there to pull an unfired case out of the chamber.

An advertising claim for the HK P7 (another delayed blowback pistol with chamber flutes) was that the extractor was unneeded for the reasons stated above. I'll bet that ejection is not entirely reliable in the G3 rifle without the extractor to serve as a pivot point for the ejector.

 

[Tony Williams]

At least one early blowback pistol was made without an extractor, and the cartridge case had no rim or extractor groove (6.5mm Bergmann M94 from 1894), but it was soon realised that the only way to unload an unfired round (or, much worse, a misfire) was to poke a stick down the barrel. As a result this was soon replaced by the M96 version with a conventional extractor and a modified rimless case.

 

[Slamfire]

The idea was patented in Italy, sometime around WW1 IIRC, and was used in at least one interwar Italian MG.

My source was "German Military Rifles and Machine Pistols 1871-1945", by Has Dieter Gotz

Pg 218 "In studying Soviet aircraft machine guns that the Legion Condor had captured in the Spanish civil war, the Mauser designers had noticed the chambers. Grooves had been cut in the walls of the bed. "gas-release grooves" as we say today.”

This source may be wrong, or there may have been parallel discoveries going on.

 

[max popenker]

there may have been parallel discoveries going on.

Well, Soviet ShKAS aircraft MG indeed had a fluted chamber to assist violent extraction at 1800 rpm, but ShKAS vas designed during early 1930s and Agnelli (that Italian designer) patented fluted chamber in 1915. It was used in SIA M1918 LMG and then in FIAT-Revelli M1935 MMG, among others.

Interestingly, in Russian specialized gun literature fluted chambers almost invariably called as "Revelli grooved chambers" - don't know why.

 

[ceetee]

The risk is not of the extractor ripping off the rim, though. With blowbacks, the extractor plays no part in the case leaving the chamber, that is all down to internal chamber pressure. The extractor is basically there to pull an unfired case out of the chamber.

Okay, that's all great in theory... but then why does everybody say you can't use non-NATO brass in CETME's and G3's? I've read that NATO-spec 7.62x51 brass is thicker than civilian loaded .308, and that if you try to run .308 through your CETME, sooner or later you'll case separations, since the non-NATO brass is weaker. If the case gets extracted totally by the recoil energy from firing the round, you should never have a case head separation.

 

[BigG]

If the case gets extracted totally by the recoil energy from firing the round, you should never have a case head separation.

I don't think it does. The flutes reduce the amount of contact between the brass and the steel chamber and allow it to be extracted without tearing the case rim. The "gas floating" stuff is hype and BS. They use that to explain the black soot. The breech is really opening when there is high pressure, imho.

 

[Tony Williams]

Okay, that's all great in theory... but then why does everybody say you can't use non-NATO brass in CETME's and G3's? I've read that NATO-spec 7.62x51 brass is thicker than civilian loaded .308, and that if you try to run .308 through your CETME, sooner or later you'll case separations, since the non-NATO brass is weaker. If the case gets extracted totally by the recoil energy from firing the round, you should never have a case head separation.

The separation occurs when the case body is sticking to the chamber walls, but the chamber pressure is also forcing the base of the case (and the bolt) backwards. If the case is not strong enough, the base will get torn off the body and thrown out the back, leaving the case body still stuck in the chamber (and the user with a big headache). So this has nothing to do with the extractor tearing the rim, it's all down to the chamber pressure overcoming the strength of the case.

With a stronger case (in conjunction with a fluted chamber), the backwards pressure on the case is able to overcome the 'stiction' and force the whole case out.

 

[Slamfire]

I don't think it does. The flutes reduce the amount of contact between the brass and the steel chamber and allow it to be extracted without tearing the case rim. The "gas floating" stuff is hype and BS. They use that to explain the black soot. The breech is really opening when there is high pressure, imho.

I don't own a roller block, but in a Brassey's military book, and with the brass I have picked up at the range, the flutes go all the way up to the throat. All the printed discussions of the functions of these flutes call it gas lubrication. The cases I have picked up are definitely higlhy sooted in the flutes. The soot must be powder residue, and the purpose is to reduce the contact friction between the chamber and the walls.

And the breech is unlocking very early, and there is a high residue pressure in the breech. Which is why the flutes are necessary.

The problem is not too little breech friction, as some would claim, but rather too much breech friction. These mechanisms would work much better if the case was a frictionless gas seal.

Totally new to me that these flutes were in an operating machine gun like 1915.

 

[AZ Jeff]

why does everybody say you can't use non-NATO brass in CETME's and G3's? I've read that NATO-spec 7.62x51 brass is thicker than civilian loaded .308, and that if you try to run .308 through your CETME, sooner or later you'll case separations, since the non-NATO brass is weaker.

My experience says that's not true.

I used to reload the (commerical)brass for my HK-91 quite sucessfully. The grooves imparted by the chamber flutes WERE DEEPER than those on military brass, and could not be totally removed by resizing. This limited case life to some extent.

I found that using fast burning propellants was key to keeping case swelling (and flute engraving) down to a minimum. The HK/CETME roller lock system opens at a constant rate, REGARDLESS OF CHAMBER PRESSURES. The key to limited swelling is to keep chamber pressure as low as possible when the case becomes unsupported following roller retraction. This can only be done by using propellants with fast pressure curves.

 

[Southern Raider]

I was thinking a bit more about the roller lock action and extraction, and came to the conclusion that an extractor is very important in the HK G3/CETME design.

If you examine the roller action, you'll see that as the bolt is pushed rearward, the carrier is also pushed rearwards, but at a multiple of this rate. This is very easy to see; just look at the bolt to carrier gap as you cam out the cocking lever. It grows, thus the carrier moves faster.

When the rollers disengage from the trunnion and the locking beak grabs the rear of the bolt head, the bolt and carrier must now travel at the same rate. The bolt, being lighter and moving slower, is rapidly accelerated to the velocity of the carrier. Due to conservation of momentum, the group travels slightly slower than the original carrier velocity, but the carrier prevails due to higher mass and velocity.

This is where the extractor is needed. The case must also be accelerated rapidly to be ejected properly. I'll also wager that before this happens, the flutes have largely vented the chamber, so we can't depend on gas pressure to properly expel the fired case.