In the novel, Earth did in fact blow up the cannon, but the rebels (or rather, their supercomputer) had prepared for that eventuality and built a second cannon in secret to continue their bombardment, and that lasted long enough for them to get their independence.
So yes, the cannon itself would be a weak point in this strategy. Unless your first targets were specifically every spaceport on the planet, you would be vulnerable to a counterattack. That would have gone against their strategy to only hit uninhabited areas to minimize casualties, but in a different context, that would be the smartest opening move.
I'd think building a rail launcher in secret on the moon would be a problem...rarefied environment with not the same kind of cover as earthbound operations.
but a lot of the practical problems in using the thing have been brought up already in the thread
there's the problem of 1 launch and you've given it away
and since it's ballistic flight, the firing solution can be RE'd -- which they've been doing b/c you are gun-mailing commercial payloads already
so the launch is vulernable to countermeasures like daedelus detonations (you dont have to destroy the thing, even altering its speed is throwing off its target)
and like there isn't going to be stuff stationed at L1? hell, that's probably where translunar flight control is for those packages luna is sending
The other big problem that already got covered is with a long-transit ballistic flight, your aiming precision is going to be pretty low-- someone brought up a good point - when we send probes etc, they go into orbit then de-orbit. it's not just flying at the target
I mean there were a few impact probes back in the 50s but the objective there was "hit the moon", they were more interested in if they could get the rocketry to work at all (and the failure rate was high high high)
So it's basically a WMD terror weapon is all you got.
I'm not sure Earth would even have to conter-attack in the foreseeable future. Tell em to have fun, there won't be resupply and when their bones turn to Jell-o dont come crying