ZCI Lightweight Nylon Piston Head

This thing is solid. The o-ring that it comes with provides a perfect airseal. My compression setup is a polished brass m4 cylinder guarder cylinder head with ARS sorbo Teflon tape and SHS m4 o-ring nozzle and a SHS 15 tooth piston. Again a perfect airseal the reason for the 4 stars is that it won't fit #14 o-rings because they are too thick. You'll have to sand the o-ring chamber quite a bit to get one to fit.