Underslung Rail

An underslung rail allows various weapon accessories to be attached to the underside of a firearm.

The rail was implicitly used in Battlefield 2, Battlefield 2142, and Battlefield: Bad Company 2 for their respective assault rifle attachments.

Battlefield 3
The underslung rail is formally introduced, and used for Primary Attachments. It doesn't actually do anything, merely a "placeholder" attachment, so to speak. It is available for assault rifles, carbines, light machine guns, and sniper rifles. A bipod or foregrip can be attached to these weapons once unlocked.

If the player has unlocked either the M320 GLM or the M26 MASS for the Assault kit, selecting the rail will allow the player to switch to these gadgets faster. Otherwise, selecting the bipod or foregrip will cause the player to use the gadgets as a stand-alone weapon (increasing weapon switch time). Although, some weapons may not allow specific attachments onto their underslung rail, i.e., the AN-94 will not allow the M26 MASS, nor will the G3A3 allow attachment of the M320, but will go vice-versa for both mentioned weapons. Using the Ammo specialization in conjunction with an attached grenade launcher doubles its ammo, from a total of four to a total of eight, including the loaded round for both numbers.