Making S.B.R. - Semi

Discussion in 'Firearms' started by gunsmoker, Oct 31, 2007.

  1. gunsmoker

    gunsmoker Lawyer and Gun Activist

    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.
  2. USMC - Retired

    USMC - Retired Active Member

    I would venture a guess and say the gas port is moved back as well. Just my guess though...

  3. Jmark

    Jmark Active Member

    I think a lot has to do with the make and action of the gun. I know with AR's the shorter the barrel the higher the risk of failures. This is one reason for the use of the gas piston uppers. An 11" AK already exists and are not all that hard to get. As far as shotguns go I don't really know, though I have seen that many folks are cutting down Saiga's.
  4. ptsmith24

    ptsmith24 New Member

    Maybe use a lighter recoil spring so the action takes less force to cycle?
  5. WGTactical

    WGTactical Member

    If you want high reliability, go gas piston upper. These respond more quickly as the piston is right there at the port. Very few SBRs we've seen are considered reliable without the use of specific types of ammo. As was mentioned earlier, the port location relative to the end of the barrel is quite important.
  6. Firearmz

    Firearmz New Member

    The piston driven guns are making a showing but still have not got to the point that their reliability is worth the cost. I have an acquaintance of mine that runs a training center and almost every piston driven gun has failed to complete a class. The gas guns are still more reliable in the long run if maintained properly with high round counts. Even down to the 11 1/2 inch guns.

    For info, the POF and Bushmaster are the very same gun made by POF. In the piston driven guns the LW is probably the best on the civilian market and the HK416 is rated as over all for best for quality and reliability. Both guns are in excess of $1500.00 and the HK not being available yet to the public uppers alone are going for $5000.00.