>>31159
>there is NOTHING THAT CAN STOP a million-ton rock hurtling in from space
True enough, but for the same reason it's incredibly hard to get it moving in the first place.
The change in gravitational potential energy moving an object from the surface of the Moon to L1 is something like 3MJ/kg.
Say you use a chemical gun. Combustion of H2 with O2 has an energy density of about 16MJ/kg, so ignoring questions of efficiency (e.g. heat loss), you're looking at around 200 kg fuel per ton of rock. An object dropped from the Moon has energy 15 times its mass in TNT equivalent. Together, fuel is worth roughly 75 times its mass in TNT equivalent rock delivery.
The US nuclear arsenal (in theory) could field something like 500-700 Mt of warheads. That works out to the equivalent of up to ten million tons of fuel for your gun. And you have to produce all of this on the Moon. And this ignores the materials required to actually build your guns.
A railgun has efficiencies of, say, 30% at converting electrical energy to kinetic, so that's 10MJ per kg of rock to L1, and thus 670 GJ per kiloton TNT equivalent. In other words, your yield is only 6kt per kt of energy input, and what you pay for that increase with is the massive amount of infrastructure to support many (you need to be able to launch many in a short time, after all) impossibly large guns on the desolate Moon, and the massive power supply they would need.
And launching it aside, think of what it would take simply to carve out a thousand ton rock and move it to the gun!
