No Mounted Machine Guns in Battlefield 3
DICE's Senior Gameplay Designer, Alan Kertz, confirmed via a twitter response to a fan that stage-mounted machine guns are not in Battlefield 3. In previous Battlefield games there were areas on the map where players could find a mounted machine gun and use it to protect an area. However, since most weapons now feature a bipod, there is no longer need for this. The bipods allow players to mount guns on most surfaces and take over the role the mounted machine gun once played.
Alan Kertz responded via twitter, “Every guy with a bipod is a turret. It’s otherwise unnecessary.”
According to the Battlefield 3 blog, "All Light Machine Guns in Battlefield 3 will have the ability to be mounted on most surfaces with a bipod, and so will most rifles as well (and sniper rifles, for that matter).”