For: JBU Aluminum piston head with bearingsProduct Information:
It is made of aluminum so if it breaks, you're in trouble - be sure you inspect your gun after every 10000 shots or so. You should have a reinforced gearbox if you are using this piston head. p=mv, the faster this goes, the more momentum it will have that is transferred to the gearbox shell. Very durable piston head and the ball bearing acts as a spring tensioner (higher fps).
With my guarder sp120 spring, ball bearing guide, polycarb piston, and double o-ring cylinder head, I get 400 fps with an m4 ported cylinder when i would have been getting 390.
If you like everything metal in your gun, get one of these things. Don't go full auto often with it or it could crack your box (but i do it and risk it anyway.) After 30000 shots, the front of this thing is still unmarked.
aluminum (screwhole threads wont strip)
ball bearing (adds tension to spring)
holes to allow for o-ring pressure