I'll stick with the pre-Rocket "pretty well" summation by Ben and Pipes. Little bitty ejecta go one way damn skippy; big ol gun don't come back all that fast but since it's usually about 96 times as heavy as ejecta it comes back for real no matter how you "perceive" the event. That's a Newtonian moment; it's equal and oppo; it's recoil. All that business from Rocket about battering and hammering and is the gun "unitized" thru the stock head, etc. is fine but recoil is sufficient as an immediate cause. Pressure may be the primum mobile in putting a lot of expanding gas between piston and cylinder head but all that stuff moving is enuf to explain mechanical wear without introducing Caspar the unfriendly ghost in the machine.

jack