From: Icuras08 | #005 Are we going to have to do something to unlock them at the shop?
you'll have to find the unlockable weapon's blueprints hidden in the single player levels. They're in the normal levels as well as secret levels you have to find through codes on stuff found with the ASE. I'd be surprised if you couldn't unlock them through multiplayer as well, but they haven't mentioned anything about that yet.
Playing: Batman AA, Borderlands Want: Golden Sun DD, Killzone 3, Batman AC, Infamous 2, Brink, Conduit 2, AC Brotherhood, and Borderlands Robot Revolution dlc.