A semi-automatic or automatic rifle is provided. The semi-automatic or automatic rifle includes a receiver having an integral hand guard portion, a barrel having a bore. The bore is removably connected to the receiver, the hand guard portion extending over and surrounding the barrel, an indirect gas operating system has a gas block and a piston. The gas block has a cylinder, the gas block is fitted to the barrel, the cylinder is in communication with the bore. The piston has a piston end and a striking end, the piston end is fitted to the cylinder and has a bolt carrier having a striking surface. When a cartridge is fired, gas displaces the piston end and causes the striking end to strike the striking surface displacing the bolt assembly, wherein, the cylinder and the piston are together removable as an assembly from the firearm without disassembly of the rifle.