Here is the thread for that. I was wondering if there was also a mission or accomplishment that was supposed to unlock the Concussion Railgun like it was for the Superlight.

Suggestion: You can buy Concussion Railguns if you completed the mission and manufactured a Concussion Railgun.

+1, or at least remove it from the shop

bump. I might actually try to figure out a way to use the conc railgun if it was purchasable from N2.