[chrispenycate]

Mass driver, yes, and the source of one of the few technical errors I caught Heinlein in; they didn't require steel shells, any reasonable conductor (like aliminium, for example , which could easily be extracted from the lunar regolith) would have sufficed. (he also had a tendecy to ignore the law of conservation of momentum occasionally, when it suited his purpose)
And I think the "undergroud"was as much for thermal reasons as radiation screening; the moon's day and night being so extended, the heat in the afternoon and the cold at night will both be extreme enough to make any Earthly desert seem very clement. A few metres of rock will reduce this cycle considerably.

And something coming in in free fall from lunar orbit would(if it weren't for air friction and the tendency to vaporise on the way down) arrive at more than ten kilometres/sec – that, for a few tens of kilogrammes is a lot of kinetic energy; baby dinosaur killer. No real advantage to chemical explosives after that, and they'd only render the missile more vunerable.