Hey potato launchers! (or whatever y'all call each other! XD) I've been researching for a potato gun project at school and need some help. I can't use propellant so just air pressure for now. So far my materials are: Tank (4 in. pvc to 3 in. pvc) Valve (3x3 in.) Barrel (3 in.) Glue/Primer (8 oz each...