The Pen BBS 500 has a piston on a rod that takes up half the barrel. Rather than turning a knob, there is an extendable rod that stows away inside the shaft that supports the piston. The rod is unscrewed from its storage position, pulled back, and then twisted again to lock it to the piston shaft 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.
