Pressure produced from the ignited propane pushes the piston forward, which opens a poppet valve and releases exhaust gases into the barrel. The piston continues forward under its own momentum, bounces off the spring and returns to its starting position. It is unlikely that the self-loading version described in the patent will be produced, due to excess heat production. See also Paintball Paintball marker Stock paintball Tippmann Woodsball External links Propane VS /Nitro Comparison (PDF) Paintball markers