Battlefield 3 developer DICE is aware of an issue with the Back to Karkand DLC on PlayStation 3 that occasionally borks a number of weapons and is working on a fix.
A post on the Battlefield Blog explained that the studio is aware that the QBU-88 sniper rifle and MG36-LMG are "not acting as they should".
"We have identified the problem and are working on a fix. We will keep you posted on our progress!"
Judging by user feedback posted on the game's official forums it seems that the weapons aren't appearing on screen for some players once they've been unlocked.
The Battlefield Blog announcement notes that the weapons should work as normal on the base game maps.