You could always go with a something a little less lethal but annoying enough to get the cats to find a less hostile home, like a paintball (delivered via paintball marker or slingshot). The drawback is that you've got to be able to see your target and the odds are the ferals aren't just lining up on your fence...and besides that means you've also got to get up in the middle of the night and go after 'em. Trapping them (or much better, having somebody else trap them) is a much better solution IMHO.