http://www.weaponsguild.com/forum/index.php?topic=41925.0

notebook: Becker blowback system

TRX
July 06, 2013
The Chinn machine gun design book talks about "forward ignition", where the mass of an open bolt moving forward provides enough resistance to keep the heads from blowing off cartridge cases. An example of this is the 20mm Oerlikon machine gun.

Over on this page: http://www.guns.connect.fi/gow/2030.html I found a description of the "Becker system" circa WW1. The text says:

"The SSG36 used a Becker principle of bolt head following the rebated cartridge base deep into the chamber. After firing, the case and the bolt could safely back off from the chamber without immediately exposing the base to explode under remaining chamber pressure."

I dug up a .pdf of a 1943 Oerlikon training manual. I noted these features:

1) the cartridge went deeply into the barrel - about the length of the case from base to shoulder.
2) the case head was rebated so an extractor to grab it
3) the barrel was sprung and could travel several inches backward like a "long recoil" action
4) it used the "forward ignition" principle

Not all of these are applicable to a pistol or SMG, but if you had a straight blowback design that simply required too much bolt mass to keep from beating itself to death, #1 and #2 might be useful.

The major downside for us would be extraction. Rebated case heads are rare in pistols, but if you wanted a tube gun in .41 or .50 AE you'd be set. Most blowback designs will work just fine without an extractor; indeed, some pistols were designed without them. This problem here is extracting an unfired round to clear the gun; having to drive a live round out from the muzzle would be a hassle.


TRX
July 07, 2013
Throw in another idea: use an "auxiliary chamber" like some of the .22 1911 conversions. The cartridge case doesn't have to move in the chamber until the auxiliary chamber hits its rearward stop.

The auxiliary chamber adds complexity, but still far less than any locked- breech system.


TRX
July 30, 2013
I give you: the Cobray Terminator

It's an open bolt, single shot 12 gauge shotgun. No locking mechanism. The forward motion of the bolt provides enough resistance to keep the shell from coming apart when it fires.

http://forum.gon.com/showthread.php?t=306849

http://forum.snipershide.com/firearms-sale/196684-cobray-terminator-12ag-open-bolt-shotgun.html