Abstract: An inexpensive generally retrofitable drop-in cartridge-unit of light-weight pneumatic-cylinder design for replacing the conventional metal/compression-spring member housed within the frame of a gun, thereby providing significantly smoother, quiter, more rapid, and more reliable cyclic-action upon firing of the gun. The preferred embodiment cartridge-unit is internally configured whereby a near constant spring-load rise is provided as the bolt-action slides aftward, thereby effectively spreading resultant recoil-action inertial-kickback more evenly throughout the bolt-travel. Hence, substantially reducing disturbing kickback, enabling a more rapid succession of shots per given shot-grouping owing to the shooter's improved recovery-time. Various iterations are set forth, including those of single, double, and tripple telescopic-action, and a hybrid/metal-pneumatic variant, as well as generic-variants providing manual and automatic pneumatic-pressure adjustment.