Someone mentioned before that you could probably set them off by tossing a fire cracker from a distance, or something similar. Just make something go bang right in the middle of them and let them all do the work.

I'd not try it, frankly I think it would be terribly irresponsible to want to start gunfire in the middle of a city, since there's way too much chance of innocent people getting hurt. But, hey, I wouldn't cry about it if it did happen.