You're talking about a mass driver, the likes of which has been featured in several sci-fi books (e.g. The Unreasoning Mask, by Farmer) and vids (e.g. Babylon 5). I agree -- This is the scariest potential military application of a moon base.
...though his invention worked superbly -- his theory was a crock of sewage from beginning to end. -- Vernor Vinge, "The Peace War"