Tried and tried to figure out how to unlock this for one of the assignments. Had no idea how to even unlock it. Apparently I have had it for some time...
From the BF3 Wiki
"The M320 is an Assault gadget in Battlefield 3's multiplayer. The M320 is an underbarrel attachment when the assault rifle's second slot is left empty (on the 'Underslung Rail' option), but is a standalone weapon when the second slot on the assault rifle is filled. "
Enjoy.