Even a revolver, that loses pressure quickly, gains higher velocity with a longer barrel. Using the same cartridge in a 2 1/2" barrel, and a 6" barrel, you will see gains in velocity in the longer barrel.
I don't see how the pressure can be loss. If there is no pressure after leaking around the bolt, there is no pressure to drive the bullet through the rifiling. And you should have a stuck bullet in the bore. Unless the bullet has already exited the bore. It all happens very quickly.
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