Piston Return Spring for FN C1 Rifle The FN rifle is a gas operated semi-automatic only, magazine fed rifle. It uses short piston stroke gas system with gas piston located above the barrel and having its own return spring. After the shot is fired, the gas piston makes a quick tap to the bolt carrier and then returns back, and the rest of the reloading cycle is commenced by the inertia of bolt group. The gas system is fitted with gas regulator so it could be easily adjusted for various environment conditions, or cut off completely so rifle grenades could be safely launched from the barrel.