the slidable mass being a compressor piston

Full Title

Compressed-gas guns, e.g. air guns; Steam guns > characterised by the supply of compressed gas > having a piston effecting a compressor stroke during the firing of each shot > the piston being spring operated > having an additional slidable mass moving in the opposite direction to the piston, e.g. for recoil reduction > the slidable mass being a compressor piston