I've built...
A railgun: It's about 6500 joules, running off of a capacitor bank connected to some batteries used in fighter planes I bought on a govt. auction site. Theoretically portable, punches holes through a hood I bought at a junkyard.
Coilguns: Quite a few, most built using disposable cameras. The largest uses an old Super Soaker as the body and can punch through a glass Coke bottle.
Rocket Launcher with 8 shot revolving magazine, features a gas seal system: Made from PVC, originally used to shoot mortar shells for firework worthy events, later modified to shoot home made model rocket based rockets, no warheads yet cause it's illegal.
Plunger Cannon: Made from PVC and uses air pressure to fire plungers in homemade sabots. Max distance: 350 ft. Also fires homemade paintball grapeshot rounds quite well. Could probably be made to fire homemade ball bearing grapeshot rounds, too.
Super Flare Rifle: Shoots flares, has a removable magazine. Bolt action. Why? Cause it's awesome on the 4th of July.