Introducing a 3D printable fastgun piston. Available in three convenient sizes: 1/2, 5/8, and 3/4 inches tall. They have a 3/8" wide base and the piston rod is the standard 3/16 dia.
Printing guidelines: I had good success printing at 0.2mm layers. I think both finer or thicker layers would work good as well. I recommend 100% infill and slow speeds.
After printing I like to smooth them up with a file and then give them an acetone wipe.
Their height can be easily modified with a utility knife when building guns.