http://www.weaponsguild.com/forum/index.php?topic=44867.0

Retrieved: 12/05/2013


TRX
November 13, 2013

CETME/G3 trunnions, bolts, and carriers are reasonably affordable compared to current AK and AR prices. Locking pieces are inexpensive, and the delayed- blowback system works with a wide variety of cartridges.

The problem with adapting the bits to a project gun is that they require a fluted chamber. Grooves run the length of the chamber, to just ahead of the case mouth, and gas runs back along the case, reducing its grip so it can be extracted while under pressure, instead of just ripping the extractor groove off. Fluted barrels are expensive. Fluting can and has been done DIY, but that's a sizeable project all its own.

I think I mentioned somewhere before, there was another gun that ran into a similar extraction problem - the AMT Automag II. It was in .22 Win Mag. The thin .22 case was intended for bolt action rifles, not blowback pistols. Larry Grossman, the designer of the Automag II, also used gas to float the case, but he took a different path - instead of fluting the chamber, he used a drilled chamber insert. The inserts could be made on a screw machine and pressed into the barrels; much simpler and cheaper than having flutes cut or barrels hammer-forged over a fluted mandrel.

The AM2 design is about 30 years old, currently being made by High Standard, and though you'd think the annular cavity around the chamber insert would get gunked up, it doesn't seem to have been a problem in service.

The nice thing about the chamber insert is, it can be done by anyone with a lathe and a drill.

There is a long thread about the insert, and how it works, on Ian's Auto Mag forum:

http://www.amtguns.info/

What would the hole size and spacing have to be for a .308, or 7.62x39, or other cartridge? You'd have to find out by trial and error. AMT welded their inserts in, but you could make it a snug fit and use a screw or pin to retain it while working it out.

Now, the G3 carrier assumes a "tube atop a tube" layout. But there's no reason you can't move the spring behind the carrier and saw off the upper story. You'd probably need an AR-style "buffer tube" or a really long receiver to fit everything.

The Voices keep suggesting a PPS-43, which has a really long receiver, perhaps chambered in 7.62x39. One of the more annoying Voices, whose name might be Alvin, further suggests a 75-round drum would be "far out."

The pictures below are from the thread referenced above: