SpudFiles

could someone explain to me how a piston works.

Same way a diaphram works... but instead of it bending causing the center to move, the entire piston moves bringing the center with it.

Air enters behind the piston. it pushes the piston foward until it seals with the barrel. Air escapes around the piston into the rest of the chamber that surrounds the barrel. Since the pressure equalizes the piston remains sealed on the barrel because there is equal pressure on both sides. Then from the rear (the side the air entered through) the air is vented out, therefore the pressure is no longer equal and the greater pressure on the opposite side slams the piston back opening the barrel and all the air rushes out and pushes the projectile out.
Thats assuming it is coaxial.

i think i understand so you have a T with a piston in it. you put pressure in the back of the T forcing the piston into the barrel but opening the chamber. when you let air out the the piston moves back... i get lost about there does someone have a picture of a website that explains with pictures

when you let the air out from behind the piston (the pilot area, just like in a phragm launcher) the piston will move back because the pressure in the chamber is greater than the now empty pilot chamber, when the piston moves back, it uncovers the barrel, and all the pressure in the chamber will go thru the barrel

oh i got thank you

Go here for a visual explanation if you are registered for spud tech forums: http://forums.spudtech.com/topic.asp?TOPIC_ID=7432
Pretty hard not to understand watching that.