Ok, so Co-Op in BF3 has some unlockable guns by getting exp in it. It sucks though cause most games are 8-12k EXP.
So, you need to go through all of Co-Op and unlock "The Eleventh Hour". It is the FINAL CO-OP mission.
Load up the mission on easy setting, then proceed through the level, you'll disarm a bomb then fight out of the subway. When you reach the street you'll see police pulling up to a building. If you stay outside behind all the cop cars you can keep shooting the bad guys that come running out of the building for 100 XP a pop. If you run out of ammo you can run back into the subway to a near by Ammo Crate and get more.
This method yields 150k-200k+ XP an Hour, and you only need 440k to unlock everything.
So get a buddy and get your guns before it gets fixed.
Also I can show anyone how to do this on PS3 just send me a message.