I don't have a spud gun, but I do have a sling shot that can shoot just as powerful. Its made of eight strips of !0' long latex surgical tubbing. The ends of the tubes are tied to clips so you can wrap it around an object and hook it onto itself. I've managed to launch a party balloon full of water (frozen so it wouldn't pop). You have to be careful when using it, sometimes the slingshot catches whatever its throwing and hurls it back at you.
My friend has a spud gun, It uses compressed air and a butterfly valve to work. Normally we shoot the barn or into the cow pasture.