The Pen BBS 500 has a piston on a rod that takes up half the barrel. Rather than turning a knob, there is a rod that stows away inside the rod. The rod is unscrewed, pulled back, and then screwed into the actual piston rod in the extended position. Pen BBS added another feature. Instead of depressing and pulling back on the syringe type piston filler, you only press. There is a spring that stretches as you press, and when you release the pressure, the piston retracts on its own.