I've been working on a Sentry Gun project lately. Basically, three SG90 Towerpro servos, connected to an Arduino UNO R3 under the command of a computer automatically aim and fire at movement detected by a webcam. Since the SG90s are real small, I'm going in for the lightest airsoft handguns to mount, which shouldn't be a big problem to find. I'll be posting videos and pictures of the developement in here. All I need now is to do the wiring. I also fitted the fragile parts of the sentry (e.g. the Arduino) into a shell, with several holes for wires to pass through. I think this will come out nicely.