M16 and M4 Rail Adapter Systems The same type of rail systems provided to the US Government, the M4 and M5 Rail Assembly Systems fit standard carbine and rifle gas length rifles, that are equipped with standard delta rings and handguard caps (circle for the M4 and triangle for the M16). Rails include polymer rail covers to protect the aluminum handguards from damage, as well as your hands from the Picatinny rail. Drop-in installation does not require the removal of the delta ring or front sight. Rock-in similar to a standard polymer handguard. Retaining mechanism tightens up the Rail Assembly System once installed, so that you can attach laser aiming devices and be confident in zero Mfg: Brownells