A gas combustion-powered apparatus includes a piston chamber housing a driveable piston, and a combustion chamber having a generally flat wall assembly and a cup-shaped wall defining at least one combustion volume therebetween. The cup-shaped wall is moveable in relation to the piston chamber, and has a generally flat portion opposing, and generally parallel to, the generally flat wall assembly. An ignition source is in operable relationship to the combustion volume, which can ignite a combustible gas within the combustion volume. The piston forms at least a portion of the generally flat wall assembly when the piston is in an undriven state.