Piston conversion kit keeps your M4 carbine running cleaner and longer. Piston systems are cleaner on the operating system because the hot dirty gases are expelled near the gas block instead of inside the upper receiver. Piston kit replaces the front-sight base, handguard cap, handguards and bolt carrier on your stock upper. Piston is hard-chrome plated and is removable from the front of the gas block, and the bolt carrier includes an impingement key for maximum strength. Works on semi-auto carbines only, with 14.5" barrels or longer, and includes assembly instructions. Some gunsmithing required.