For those trying to make piston interrupter cannons, rivets that can be used as an interrupter can be found here http://www.mcmaster.com/nav/enter.asp?pagenum=3201 or search for solid rivet. available in stainless, aluminum , etc. also you can search for your appropriate springs there or at www.smallparts.com As far as tubing, both rigid and flexible (along with the brass fittings) are available from www.mcmaster.com, www.smallparts.com, http://www.onlinemetalsupply.com/
Note also that #8 2.5" round head stainless steel wood screws with the treads cut off make good pistons. You can order them from small parts also. Also #8 nylon screw insulators make a good spring buffer- the short ones (1/8"?) don't require trimming.