Throw a huge rock back at them and say it was the gun discharging (if they do that, im not a gun person ) Or the more sensible thing is to just ignore them. I know its damn hard to ignore the sound of a gun shot, but they get their jollies from seeing people be scared of them. If it was me, I'd just give them a "wtf, are you retarded? I feel sorry for your mum" look and carry on.