A piston assembly is provided for a gas-operated firearm of the type having a chamber and a barrel. The piston assembly includes a gas expansion housing and a piston mounted within the gas expansion housing. An annular recess is formed in the outer wall of the piston to receive exhaust gases diverted from the barrel upon firing of the firearm. At least one longitudinally extending groove extends from the annual recess to the head of the piston and forms a pathway for diverting the exhaust gases to the head of the piston. During firing, pressurized gases are diverted into the annular recess and expand longitudinally from the annular recess to the piston head, whereupon the pressurized exhaust gas drives the gas piston rearwardly along the housing.