Blowback Centerfire Rifles--Why Not?

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Sep 16, 2003
St Louis/Mo/USA
I have wondered this for a long time, and pondering one of the XM8/new mil rifle threads it came up again--why are there no centerfire blowback semiauto rifles? Or are there? I have not ever seen any....

The main reason I see quoted is that "the weight would be excessive", but somehow in my imagination that isn't true: if you can build a locking pistol like a 1911 that fires a ~450 ft-lb caliber, then to achieve the same buffering, a rifle in the 1300-ft-lb area would need a slide about six-to-eight times as heavy as a 1911's barrel and slide. That sounds like a lot, but it's not when you consider that you would be able to take a gas-operated rifle and remove all of the gas-system components, and the receiver could be built far simpler and less-costly as well. And even if it did turn out to weigh a bit more than its gas-op counterpart, you'd still have a large reduction in parts count, considerably cheaper/simpler manufacturing and (as I see it) a considerable boost in operational reliability as well. Does anyone build such guns at all? Have they ever tried?
In a blowback, the force generated by the buring powder is balanced by the mass of the breechblock. You can calculate the weight needed to produce a blowback for a given cartridge. For a .30-06, the breechblock would have to weigh about 27 pounds.
I think the 1911 is recoil operated, not (delayed) blowback.

For a 7.62x51 NATO rifle in blowback configuration, the bolt would need to weigh around 31 pounds.

However, there is the roller locking delayed blowback system (explained in detail in the above link) which is used in the CETME and G3 (and in others which aren't rifles, such as the MP5).

Also, France's FAMAS uses some sort of lever for its delayed blowback system.
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The G3 / CETME actually IS an ingenious blowback design, but it is kinda of the Lone Ranger amoung modern arms, for reasons given above.

DUH! Warbow already said THAT!
The G3/CETME uses a few tricks to make blowback work...

The chambers are fluted, to prevent the brass from sticking and tearing open as the roller-delayed action opens. The roller-delay itself is a modification to a regular blowback system, to allow chamber pressures to drop low enough for safe and reliable operation, without a 27-pound bolt or bolt/bolt carrier.

Then there's pressure itself. A 9mm or .45 ACP doesn't generate anywhere near as much gas pressure or volume as a .308 Winchester or .30-06 Springfield. So you can have an Uzi or MAC-10 that's blowback that works fine in an autoloading handgun cartridge, it'll run fine using a massive counterweighted bolt. (Hence the clunky size and shape of the Uzi and MAC-10, and High-Point handguns)

Normal, concealable handguns step away from the straight-blowback operation right around the .380 ACP/9mm Makarov size of cartridges.
I'd like to add that reliability of gas operated rifles is superb if they are designed with a large amount of leeway given for the effects of fouling. It also lets you use a garand/AK system of locking the bolt during each shot, which means the action of the rifle itself doesnt move until the bullet is leaving the barrel already.

As a programmer, something about having a race between the pressure curve in the chamber and the speed of the bolt extracting the cartidge makes me nervous.
The old 1907 series of Winchester semi auto's were blow back guns I believe (never having owned one) . They required anemic cartridges suchs as .401 Winself loading nd the slightly better .351 Win self loading- think trans sonic velocities with light for caliber bullets = low chamber pressures. :)
An Advanced Primer Ignition blowback does greatly reduce the weight of the bolt required because the cartridge is fired while the bolt is still travelling forwards at maximum speed. This means that the initial recoil impulse is used up in stopping the bolt before it is pushed back again. The effect of this is to smooth out the recoil impulse by comparison with gas or recoil operated mechanisms (all other things being equal). A disadvantage is that the gun can only fire from an open bolt, which does not help accuracy.

This API blowback principle was used by the WW2 20mm Oerlikon cannon and the 30mm MK 108. For this to work properly, you need specially-designed cartridge cases with rebated rims so that the bolt can follow the round into an extended chamber.

Oerlikon did make a couple of semi-automatic anti-tank rifles using the same principle. One of them, the SSG-36, fired full-power 20x110RB cannon ammo in a gun weighing 38.5 kg (85 lb).

Tony Williams: Military gun and ammunition website and discussion
DougCxx, just in case it wasn't made explicit in the above posts, the weight of the bolt that's needed to counteract the recoil in a blowback design is not a linear relationship to cartridge energy. That's why weight you'd actually need isn't proportional to the 1911's slide weight and the energy the 45ACP produces.

If you want a better idea of how much weight you'd need to put behind the cartridge look at the recoil the cartridge generates.

You'd have one hefty bolt or a recoil spring that would take a gorilla to cycle.
I'm not sure of the numbers, but this just doesn't seem impossible to me--the figures I am tossing around show you could have a straight-blowback .223 that would weigh perhaps only a few more pounds than a gas-op arm. I'm thinking a breech block weighing around five lbs, inside a <7-lb gun.

One thing I can see: as the caliber energy gets higher, there is a larger and larger weight savings with using a gas-op system--but by the same argument, at the smaller end of the caliber range, a straight blowback gun wouldn't weigh much more, and would be much simpler to produce and maintain.
Not quite what you want, maybe, but Ruger's pistol caliber carbines are "mass delayed blowback".
Kel-Tec's SUB 2000 carbines, also chambered in pistol calibers, is a simple blowback design too...they have pretty stiff recoil springs.

It'd be kinda neat to see a larger caliber (30-06 etc) blowback action.
I wonder how accuracy would fare.
The problem with a straight blowback with a powerful cartridge is that what you want is an initially strong resistance to bolt movement to give the bullet time to get out of the barrel, followed by much lesser resistance so the bolt can move back easily to complete the cycle. This is of course what a retarded blowback action like the G3 provides.

There are limits in how far you can use a strong spring because that gives you a steadily increasing resistance - not what you want. The best method is to use a heavy bolt, as its inertia provides the initial resistance, then when it gets moving its momentum will take it through the rest of the cycle.

The problem with a heavy bolt (apart from its weight) is that it can be affected by gravity if you're shooting uphill or downhill - that can make quite a difference!

Tony Williams: Military gun and ammunition website and discussion
The HK delayed roller-locked blowback design is hell on cases. Extraction starts (if ever so slightly) while the case pressure is still high. :eek: Thus the requirement for flutes in the chamber to reduce friction between the case and chamber wall. Makes me wonder why the designers didn't arrange to have the barrel recoil backward a short distance with the bolt (like a Colt 1911 or Auto 5 shotgun). This would add the barrel's mass to the mass of the bolt in terms of resisting recoil, and would allow chamber pressures to drop before opening the chamber. After a short travel a stop in the receiver could halt the rearward travel of the barrel and initiate the opening of the chamber. The barrel would only need to travel backwards long enough to allow for the pressure to drop and the case to come un-stuck from the chamber wall (maybe an inch or two?). It would take some of the sting out of the felt recoil, too.

I prefer a gas-operated, fixed barrel rifle myself. They will always tend to be more accurate. I'm not saying the HK design is unreliable, history has proved otherwise. I just think that it succeded despite it's design, not because of it.
351 Winchester S.L.- A 180 Grain bullet with a muzzle velocity of 1850 fps yielding muzzle energy of 1370 foor pounds.

.401 Winchester S.L.- A choice of a 200 grain bullet with a muzzle velocity of 2135 fps yielding muzzle energy of 2020 foot pounds,
or a 250 grain bullet with a muzzle velocity of 1870 fps yielding a muzzle energy of 1940 foot pounds

They have a 357 and a 41 Mag Rifle round velocity......Nothing to sneeze about!!!!
Plenty of power for deer......
Makes me wonder why the designers didn't arrange to have the barrel recoil backward a short distance with the bolt (like a Colt 1911 or Auto 5 shotgun).

Actually, what you would get if you applied this system to the G3 is the MG 42.

Allowing the barrel to recoil would add complication and weight, and would also not add anything to accuracy. The only short-recoil infantry service rifles I can think of offhand were the Federov Avtomat of 1916 and the US Johnson of WW2; neither were regarded as very successful.

Tony Williams: Military gun and ammunition website and discussion
The Model 1907 must be given credit as the first successful semi-automatic rifle marketed in the US. Its immediate predecessor the Model 1905 was not a commercial success as it was chambered for a couple of low velocity, ineffective rounds, the .32 S.L. and the .35 S.L.

Interesting that the .32 SL was the basis for the .30 M1 Carbine round, which did indeed have a reputation for ineffectiveness against two-legged as well as four-legged targets. Pity they didn't decide to design the Carbine around the .351 SL.

Tony Williams: Military gun and ammunition website and discussion
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