Also if you're talking about a gun, in D&D thats probably better off as something like a Necklace of Fireballs or some other use activated magic item. A crossbow isn't even going to reliably kill a cat at point blank.
But then, cats are like min-tarrasques, but far more lethal at that ECL.
 Also, I've had a few DMs that pulled crap like this. For one, I was a baby gold dragon, not a paladin. I'd read in the source material (Dragon Magazine) that baby dragons tend to be a lot more chaotic and gung-ho than adults, and so I wrote CG on my character sheet. The DM didn't think I was playing my PROPER alignment of LG, so he kept making my breath weapon burn down forests and catch other players (explicitly because, 'we don't have a battle map, so you can't aim') and gang-raped my baby dragon by a clan of ogres and hill giants to punish me for it. He then went on to make everything I did to help others backfire in the most ludicrous ways possible because apparently everyone in the world was hyperbolically Chaotic Stupid.