Quote Originally Posted by BTDT View Post
To really split hairs, inertia is a property of a moving mass only if that movement is uniform, which the piston is not - it's either accelerating or decelerating (apart from an instant before it bounces).

The important point is that it is the reduced momentum of the lighter weight piston that makes it less able to overcome opposition from the compressed air.
As momentum is a simple function of mass and velocity, it could be argued that the reduced inertia, and thus higher attained velocity of a lighter piston would give it enough momentum to compress the air.

It would be fascinating to see high speed x-ray film of the whole firing cycle, but until then we have to rely on the painstaking empirical research of you springer experts, for which I'm grateful.