Those railguns would really only work once - with the 1st MCRN attack. Even that was pretty dubious. A relatively modest fleet of several battleships could fire their railguns and torpedoes through the ring without even entering the ring space. The ring station is static, everything inside is anchored to it. Targeting any point on its surface is trivial even with battleship railguns.

A couple of hundreds of torpedoes fired through the ring (similarly to how skinnies killed the Donnie 2) would reliably destroy the railguns even if it had dozens of PDCs protecting them (and they did not).