Abstract: A gas tappet system for use in an M16, M4, or AR-15 type rifle having a detachable piston eccentrically connected to an operating rod. A piston having a plenum therein and a releasably attached operating rod uses combustion or propellant gasses to move the piston in a long stroke, preventing damaging impulses to the bolt carrier and cycling rounds for automatic or semi-automatic fire. The operating rod, piston, cylinder, and mounting clamp permit easy installation on the rifle without any modification and can easily be removed for reinstallation of the original direct gas impingement system.