Everything is SCH40 PVC:
2", 8' long barrel
4", 4' long chamber
Piston is a 3" ABS cap
Pilot valve is a 0.75" jar top sprinkler valve.
Not really much else to say because it's pretty basic and ugly, but here are some pics!
The barrel extends to the edge of the pic, just hard to see because of the sun
Piston sealed against the barrel. For a bumper I just coiled up some air compressor tubing and let that expand around the circumference. The piston weighs hardly anything so it works fine.
Ugly barrel supports but they get the job done perfectly
Im just using decal material (really thick sticker) to provide the seal with the barrel. I used a router on the the cap to get a perfect ramp shape so the air passing doesn't hit a blunt edge.
There was a pretty small gap around the piston and it wouldnt fire so I had to mess around. A combination of decal sheet and electrical tape fixed that and it fits perfect now.
All in all I'm pretty impressed with it. I don't get any honking, piston opens basically instantly. I've only had it up to 100psi, my compressor only goes up to 120psi. As for muzzle velocity I have no idea. Only thing I can really say is that it splintered up a 2x6 I shot with a potato but that doesn't really come as a surprise.
good job, and yes damage pics please! 
happy spudding.
