I have a technical question about the mechanics of how certain firearms work. If I wanted to make a short barreled rifle or shotgun out of a semi-auto long gun that is gas operated, is there a limit on how far back I can trim the muzzle before impairing proper functioning of the weapon? I know that these kind of firearms have a hole drilled into the barrel, usually about 3/4 of the way toward the muzzle, which vents some of the high pressure gasses as soon as the bullet passes by the hole, and sends those gasses back toward the action to cycle the bolt. Obviously if you cut the barrel at or behind the hole, the system can't work. But I would think that if you cut the barrel too close to the hole, even if you were still beyond it toward the muzzle an inch or two, it would still prevent the gun from working, because the amount of time that those gasses would be using the hole would be too short. Before the bullet reaches the hole, there's very little pressure going in (just a slight amount from compressing the air forward of the bullet). As the moment the bullet passes, THEN the hole gets a charge of hot, high-pressure gas. But if the bullet only moves another inch within the bore before exiting the abbreviated muzzle, as soon as the bullet clears the muzzle the pressure in the barrel will drop to nothing, and no more pushing action will be achieved. With a standard gun with a long barrel, the bullet will have several inches of barrel to travel though after it passes the hole before it reaches the muzzle, giving extra time for those gasses to push the piston against the operating rod (or whatever applies to that particular design of gun). Does anybody know how people who build NFA firearms deal with these issues when they want to make an 11" barreled AK, AR, .30 carbine, etc? Specifically, I'm thinking that one day I'd like to have a semi-auto shotgun with a pistol grip stock and the barrel cut off at about 12", for home and vehicle defense.