An M4 type automatic or semi-automatic firearm having an indirect gas operating system. A bolt assembly has a striking surface. The bolt assembly is enclosed within an M4 type receiver assembly. A barrel assembly having a bore is coupled to the receiver assembly. A gas block having a cylinder is fitted to the barrel assembly with the cylinder in communication with the bore. A piston and rod assembly has a piston and a striking rod, with the piston fitted to the cylinder. Gas discharged from a fired cartridge displaces the piston and causes the striking rod to strike the striking surface displacing the bolt assembly.