This is just the bastard child of an off-hand comment to my friend one day at school that I felt like building a marble rifle. After I got home, I was on a mission...
The barrel is 1/2" x 2' and the chamber is 3/4" PVC and elbows formed into a stock-type-shape-thingy. The valve is a small barrel sealer inside of a 1-1/4" tee, and the pilot and fill valve is a 1/4" ball valve with a male quick disconnect. Eventually, I might upgrade to a 1/4" Q.E.V. with a real-like blowgun trigger, but for now the ball valve works great. The piston itself is a D cell battery with some neoprene bolted onto the front and wrapped in duct tape. As always, I lubricated it with Hoppe's gun oil.
Right now, I can't really see where all my marbles are going. I CAN hear bushes and twigs snapping whenever I fire into the woods, though, so I can bet they're going pretty fast.
Might get a video up soon, it's too dark and I've got too much to do for now.