Delayed blowback with reciprocating weight?

[230RN]

"At the moment of firing (likely by a hammer swinging into a recess in the bottom of the bolt) the bolt recoils a short distance (2 mm?) before coming up short because of the flaps in the recesses, but that short movement sends the traveler on its way."

That seems to be the fly in the ointment. Allowing the bolt to come back ~0.080" while the cartridge is at almost peak pressure is almost guaranteed to rupture the now-unsupported portion of the brass case.

While a lot of guns have a small portion of the case unsupported in the chamber, the web of the case is thick at this point and the brass will hold the pressure. This is fine unless something else goes wrong, like a barrel obstruction or an overloaded case.

Garand, in his design(s) for what came to be the M1 Rifle, took a lot of trouble to avoid this unsupported area because it had been a source of "accidents" in prior designs, such as the Mauser and Springfield-type bolt rifles.

So now, unless I'm reading your descriptions incorrectly, you're going to allow the rear end of the case to stretch out 2 mm or ~0.080" while the front end is still grabbing the chamber walls. And your first description only talked about 1 mm (~0.040") of bolt withdrawal, which to my eye, looked like a recipe for disaster in the first place --see my previous post on this.

Might be barely OK for a low-pressure cartridge like a shotgun shell, but for high-intensity pistol and rifle rounds (which run up to 50 or 60 thousand "psi")... I don't think so. You might take note of the fact that the very high pressure 5.56 NATO round which grew out of the lower-pressure .223 Remington round, has had this portion of the brass case thickened substantially... even though both rounds will chamber and fire in the .223 chambers.

And just for "mental scaling" purposes, you might also take note of the fact that if the .30-06 had to be fired in a straight blowback action, the breech block would have to weigh about 27 pounds.

I strongly recommend you read Hatcher's Notebook, especially the chapter on the strength of military rifles and the one on headspace.

Again, all this if I am not misreading/misunderstanding your descriptions. The closest I can come to understanding or conceptualizing your idea is with one of Garand's original experimental designs, where the primer was allowed to set back in the case in order to unlock the bolt. This idea was abandoned by Mr. Garand.

Terry, 230RN

 

[Jim Watson]

We know that type of system works because the PA51 and PA53 allow the case and breechblock to set back, giving the slide a push. Just no "traveler," or "flaps."
Mr Pedersen set the distance of set back to equal the thickness of the solid case head, so there would be no unsupported brass until the bullet was gone.

I would be concerned that if you allowed movement of the breechblock, even limited to the case head thickness, that a long bottleneck rifle case wouldn't slip back, it would stretch. If you could keep it from separating, the Army wouldn't care that the stretched case would not be reloadable, but commercial buyers might.
Read about the fun to be had reloading the 5.7 FN with its mostly blowback action.

 

[cluttonfred]

One point to follow up on Terry's comments on the practicality if this type of Pedersen-inspired action, which someone on this board described as a sort of gas-operated action using the empty casing as the piston.

When you take a look at the cross section the casings of rimless cartridges for self-loading actions, you find that interior of the case where the powder is held is at half again and sometimes twice as far forward as the most forward portion of the extraction groove. In other words, the casing can shift rearwards in the chamber at least the distance from the cartridge base to the front of the extraction groove, and even a little more, without leaving the side walls of the cartridge unsupported. This only applies to cartridges that are pretty much straight near the base, of course. Take a look at the .30 Carbine round, for example.





From the cross-section photo (happens to be a tracer) and the dimensional drawing attached, it seems logical that the casing can be allowed to slip back about .12 inches (over 3 mm) with no ill effects, since the stressed sides of the cartridge would remain against the walls of the chamber and the cartridge base against the bolt.

There may be other reasons why this whole design concept is terrible, I just don't see this one as a major issue. The cartridges certainly wouldn't blow out from pressure alone and I don't see why the potential stretching that you mention would be an insurmountable problem. There is always the options fluting the chamber slightly to "lubricate" the casing with gas if need be. I wonder if anyone who has fired the R51 or an original Remington 51 can say whether or not either gun is particularly hard on brass?

 

[Jim Watson]

I never noticed anything odd about the .380 brass from my PA51.
I foolishly sold the neat little gun because it would not feed hollowpoints.
Now I don't use hollowpoints in .380 anyhow.

 

[barnbwt]

The issue isn't the case head, it's the walls. Under pressure, that brass is glued to the chamber, and any relative movement of the much stronger case head will stretch and tear the interface up inside the chamber. Result is a broken case at best, rupture at worst. In 9mm, the case is so short there is a lot less area for friction to act over. In a rifle it's nearly insurmountable without fluting, lubricant, or highly clean/polished surfaces (and even then, your initial shift must be slowed by mechanical disadvantage)

TCB

 

[rodinal220]

Winchester 1903,1905,1907 and 1910 used a blow back action with a large weight part of the bolt/carrier assembly.

 

[HammsBeer]

So in a fixed barrel 9mm pistol, what's the feasibility of an extended chamber with the case acting as the piston driving the slide back, so the bullet exits the barrel and pressure drops before the case starts to exit the chamber?

 

[barnbwt]

Pretty good, but mechanical extraction will be a neat trick --that's why the Oerlikon had a highly rebated case head. That's also why a non-rebated case like 9mm can only go as far into the barrel as the extractor claw will allow (otherwise the case head would probably be narrower, since the extra strength wouldn't be needed). Something like the R51 is probably as good as you can get for case support (well, not counting their overly-ramped and miscut oblong chambers)

TCB

 

[HammsBeer]

True, extraction would be tricky, but a spring loaded claw could sweep onto the rim as the case starts to exit the barrel and complete the extraction.

Another delayed blowback option would be a slide that doesn't fully chamber the round in the barrel, leaving it out of battery a few of mm. The trigger would release the spring loaded slide and hammer at nearly the same time. The round would be going off as the slide is moving forward, so the recoil of the case would have to fight the forward momentum of the slide just long enough for the bullet to exit the barrel.

 

[Jim Watson]

There are some guns that do not have extractors, or only need them to pull a loaded round out at the cease fire. Many a .22 auto has kept on shooting with a lost or broken extractor; and the Beretta tip-ups don't have one at all.

That's how a lot of submachineguns and some heavier automatic weapons operate; Advanced Primer Ignition. Complicated for a pistol and not great for accuracy with the slide slamming.

Tanfoglio developed the FAR system. It was straight blowback but the dedicated ammo had a thick solid head so that not case wall would be exposed until the bullet had left the muzzle. You got a round with the OAL of a .38 Super and the powder space of a 9mm P.
It worked, but if you got your ammo mixed up and loaded standard cartridges, I don't think you would like the result.

I always admired the 1920 Garand with primer setback operation. Progressive powder and/or crimped primers made that unreliable, and we know where he went from there.