US Patent 7775150 Law enforcement carbine with one piece receiver

An indirect gas operating system for an M4 type automatic or semi-automatic firearm. The indirect gas operating system has a gas block having a cylinder. The gas block is fitted to a barrel assembly having a bore with the cylinder in communication with the bore. A piston having a piston end and a striking end has the piston end fitted to the cylinder. A bolt assembly having a striking surface is provided. When a cartridge is fired, gas displaces the piston end and causes the striking end to strike the striking surface displacing the bolt assembly. The cylinder and the piston are together removable as an assembly from the firearm without removal of the gas block.