I've been working on this project on an off for a few months. Its a 7.62x25mm chambered AK front end. Its a 16" barrel with a very short piston (about 1.5") on a regular carrier. I haven't put a gas port in yet. The gas port will be just about 7" from the breech end, if you subtract the 1" approx case length, then 6" from the case mouth. I'm starting to worry that if the bullet has not left the barrel by the time the bolt rotates and begins to extract then I will get a case head rupture . I really don't want to blow this up.
I've seen a few other builds, notably PBB's build here, but that build used an AMD length piston and 5/32" gas port (.156") and a carrier which has been significantly lightened, recoil spring reduced. the other reference point I have is Gnarlytoe's build:
The AK Forum :: Log in
which uses a krink piston and an 20" barrel and a .205" gas port, recoil spring reduced. I can't seem to find a reference for the length of a krinkov piston, but it's longer than 1.5" by appearance anyway.
Is there a calculation for timing an ak barrel? Chamber pressure, port size, port distance from breech, carrier weight, and barrel overall length? there are a ton of variables! Also, bullet acceleration: if a tok round is going 1400fps from a 4" pistol barrel, then at the 6" gas port mark (1400 fps x 12" = 16800" per second. 16800 / 11 inches of remaining barrel = 1/1518th of a second to leave the barrel. The acceleration of the carrier backwards is going to be many times slower, + the spring's resistance, but I have no idea what the carrier weight is or what the pressure is at 6" mark, or how to factor in the gas port size.
then again I might just be making myself crazy.