Bore pressure and barrel length

[Echo9]

Let me preface this by saying that I absolutley do not plan on doing anything illegal, or helping anyone else to do anything illegal. I ask this purely out of my own curiosity, and nothing else.

Suppose you were to chop an ar-15 (or other semi-auto rifle) barrel down to the the gas block. How would that affect the pressure in the bore, as it relates to cycling the action? Would it cycle too hard? Too light?

Would the situation be different for a piston driven rifle somehow? I'm assuming the pressure would be higher at the point when gas starts gas tap, which leads me to believe that the piston would be shoving the bolt back considerably harder. Will this eventually damage the gun? For example, the spring around the piston on a Sig 550 series rifle acts as a recoil spring. The way you need a heavier recoil spring in a pistol for hotter loads, would you need a heavier piston spring?

 

[benzy2]

Bore pressure is relatively consistent at any given distance regardless of length. Its how long the pressure remains that changes most.

If you cut the barrel right at the gas block, it isn't going to cycle. You need barrel past the gas block to keep pressure feeding up through the gas tube to the bolt. If you ended the barrel at the gas block you would lose this pressure and the bolt would not get enough gas to cycle.

The situation would be the same for a piston. You need pressure pushing the piston/bolt back. If the pressure vents to the atmosphere right at the port to the cycling system you drop pressure too fast to cycle the rifle.

A carbine length system had the problem that they need to push the gas block closer to the chamber to get enough barrel length past the gas block to give enough time for pressure to build and cycle the rifle. This also means the gas port is closer to the muzzle where pressures are higher. Now you have to balance quite a few issues to get the rifle to cycle reliable without being violent. The compromise has been to error a little on the violent side so cycling has enough pressure and time to cycle reliably.