I've built quite a number of cannons for club members. I've got a couple of spares sitting around.
Required tools: small drill press, Dremil tool. cut off blade, boring bit, torch, solder.
Helpful: mini cutoff saw, bench grinder
Hard to find materials: ss rivets $40 per box, springs from small parts,
Limited source materials: small co2 hose.
Common materials: copper tube, brass tube, brass fittings