You shouldn't even need that. Come to think of it, I could make this right now. Wow, gotta love my supplies box. Will work on it today then.
My coworker and I have started work on this, using an Arduino co run pan/tilt servomotors and trigger solenoid. More when it's integrated into the Vulcan itself, as right now it is nothing but code for a microcontroller.
Indeed, you don't need that, if all you want to do is turn on the firing mechanism when motion is detected.
The idea with using the microcontroller is to raise the Vulcan or traverse it from side to side in order to track a target, autonomously. Think "sentry gun" and you get the idea.
It's true that you can do this without a microcontroller, but that's a whole lot more analog design than I have the chops for.
Holy crap. I'll leave that to you then.