Assignment (Assault Class): Professional Russian – 20 kills with Underslung Grenade Launcher
How do I use, find equip or arm the underslung grenade launcher? First off, there is no weapon called the underslung grenade launcher. You need to unlock the M320 grenade launcher, so you may have already used it and didn’t realize it. After you unlock the M320 select an assault rifle like the AEK-971, equip underslung rail, make sure your gadget 1 position is the M320 grenade launcher.
UPDATE FROM BF3 WIKIA’S M320 PAGE:
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.
The grenade launcher occupies the Gadget 1 slot, which is shared with the Medkit.
To unlock the M320 you need an Assault score of 11,000. What’s an assault score, you ask? Whether you play as Assault, Engineer, Support or Recon – you earn points for that class as you play. So that means you play Assault class until you reach 11,000 points.
How do you check your points for Assault class? The easiest way I can think of to check your points by class is to go to http://battlelog.battlefield.com/bf3/ and click the Solider Stats link, the look down the right hand sidebar for your number of points in Assault.