How about a CO2 powered airsoft gun (shoots 6mm plastic BBs, some of which are copper plated)? All the development work has been done, the supplies are readily available, and there's a ton of info out there on how to boost and adjust them to get better velocity and distance (the CO2 "gas" guns can often be boosted simply by adjusting a regulator). Many are full auto and can be adapted to shoot glow-in-the-dark pellets if you want tracers.
Since you won't need one of their more accurate historical replica guns that the collectors are after, you may be able to find something almost reasonably priced. :/ There should be a bunch of stuff on ebay.
The downside? You'll end up with plastic BB's all over your carport, which can make walking...interesting. If you need more info, email me and I'll put you in touch with my local enthusiast.
--Glenn Lyford