3647 is also making one though it is quite different (
http://www.chiefdelphi.com/forums/sh...d.php?t=147697)
One big tip is to make the diameter of the t-shirts large enough so that air doesn't leak around the t-shirt when you shoot. We were testing our mechanism and couldn't figure out why it was going a really short distance and we found out it was because our t-shirts were rolled up too tight and didn't make a seal for air to push.