11 replies to this topic

[hvac]

what do they mean when the bolt is refered to as a Blow Back or a Delayed Blow Back what is the differents?

[Whiskey Brother]

Blowback means that there is no device to delay the opening of the bolt when fired. Delayed blowback means there is some device meant to delay the opening of the bolt when fired. 1927 model Thompsons are straight blowback operated, hence the heavy springs found on these models. Early Thompsons used the Blish lock, a delayed blowback device operating under the principle that different types of metals, when put under pressure, adhere to each other. When the pressure drops, they slide over each other.

[PK.]

The 1927 Thompsons were remarked 1921 models and had the Bliss lock. They, and the 1921 and 1928 models are delayed blowback guns.

The 1927a1, M1 and M1a1 Thompsons are blowback models, having no mechanical system to delay or retard the bolt against the pressure of the cartridge firing.

Some other common delayed blowback guns include the HK’s, which use a system of rollers and inclined planes and require fluted chambers to float the case so it won’t break.

Most all 22 rf auto loading guns, as well as most common open bolt subguns are blowback.

The feature that denotes blowback, or delayed (retarded) blowback operation is that the bolt starts to move away from the chamber under direct force of combustion pressure immediately upon fireing. The delaying mechanism (if present) is not locking the two together, but slowing the process down. As the bullet is moving down the barrel, the bolt (and cartridge case) are moving away from it. This requires a balancing of the bolt mass and springs against the mass ejecta (bullet and propellant) & pressure to allow the bullet to escape the barrel befor the cartridge case entirely looses obturation and clears the chamber. The delayed blowback mechanisms allow more time for the bullet to exit the barrel and pressure to drop. It also allows greater control of the velocity imparted to the bolt in it’s rearward travel. Most blowback guns will exhibit some small remaining pressure as the breech opens, delayed guns less so.

The 1911 is an example of a short recoil, locked breech gun. The barrel and slide (bolt) are locked together as the bullet travels through and out of the bore. They move as a locked unit under the influence of recoil, not chamber pressure, until they are mechanically unlocked some time after the combustion pressure has been released by the exit of the bullet.

[TNKen]

Check this out.

http://people.howstu...achine-gun7.htm

[Stainless]

Is the bliss lock really necessary then? What happens if you leave it out, increased rate of fire?
Thanks

[PK.]

The lock is necessary in the guns that were designed for it. If you leave it out, excess bolt velocity that translates into increased force to the rear of the receiver will result and that = broken receivers.

[Stainless]

What did they do to get rid of the lock, increase bolt weight, heavier springs?
Thanks

[John Jr]

Blish damnit, BLISH...

[PK.]

The M1 was redesigned with heavier receiver sections and a more massive bolt. The M1 and ’28 use the same recoil spring.

[DINK]

QUOTE

So with autos and semi-autos, you can divide the world up into "recoil operated" and "gas operated", and you'll always be right.

Just to quibble, but that's not correct. Mini guns, chain guns, etc. are electrically operated and don't use either gas or recoil to operate. Then there's the new Metal Storm system that operates like a high-tech Roman candle.

[DINK]

You just found out about Mike Dillon's toys? He's been doing that for years. As for "common" guns, I guess it depends on who you hang out with. I have two friends (both manufacturers) who own and operate minis, so they're not that unusual, at least in some places.

If you have never seen it, order a copy of "Machine Gun Magic" from Dillon. They have footage of a shoot out in the desert that will boggle your mind. They were supposed to release a sequel shot out on the ocean somewhere, but haven't so far.

[SecondAmend]

For a few other examples of delayed (or retarded) blowback firearm mechanisms see U.S. Patent Nos. 908,631; 1,052,394; 2,365,389; 4,232,583; and 4,301,712.