I tried this with a lawnmower battery once. Melted the gun and the wire, nearly set myself on fire. Electricity is not my friend.
...Sounds like fun!

Anyway, thanks for the help, everyone! I think Ohm's law there should fix this problem; if I just put a variable resistor into the circuit before the gun, I should be able to simply adjust the amperage by the inverse of the resistance without affecting voltage, right?
And if I do that, I'll just be decreasing the amps, so it should be safe to just plug right into an outlet without the whole "undoing" thing I mentioned, I think?
If that all works the way I think it does, then it looks like I'm set! I'm not actually making it until spring break, which is a couple weeks away, but I'll be sure to post an update here when I do!