A turret. It's mounted. You spawn it and press E on it to use it.
I started this awhile ago, but never really finished it, so I figure I'd release what's done so far instead of letting it rot on my hard drive.
The script is in working order. It's a STool that lets you spawn a turret, and choose things like its model, bullet tracer and muzzle effects, and explosive bullets and stuff. The effects sometimes break unless you do effects_reload in the console, because I didn't know how to use CreateMaterial at the time I made this.
No pics because I don't care if you download it or not. It's just a gun sitting on an upside-down electrical plug.