I'm about to move out of the mountains and into my drummer's old place. He's got about an acre I think, but my range will be facing the neighbor's place. I'll probably leave a good 20ft between the target and fence, and if they still complain the cops already know they're white trash pieces of shit from all the other times they've tried to get us in trouble for no reason.
Anyway, in your situation it would depend on your relationship with the neighbors. Try to at least approach it really friendly and everything as suggested above. If they decide to be pricks about it and you've made sure everything is totally safe then they can deal with it. If you're not breaking any laws there's jack they can do about it, and if they try to do something more direct, you ARE the one with the tomahawk you know how to hit a target with.