Cylinder Head

The Cylinder head acts as a contact face for the piston during the firing cycle and channels the air burst generated through the air nozzle. A plastic or metal cap that seats inside of the front end of the cylinder on the rear end, with a smaller metal tube protruding from the front end for the air nozzle to seat over top of. Two circular notches on each side seat in the gearbox shell to locate the unit and hold it in place. 

Cylinder

A brass or aluminum tube that acts as the compression body during the firing cycle. When the piston assembly pushes forward from the back it seals up and compresses the air in the cylinder, accelerating it out through the cylinder head to fire the airsoft bb.