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 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 grenade launcher or the M26 MASS for the Assault kit, selecting the rail will allow the player to switch to these gadgets faster. Othwerise, selecting the bipod or foregrip will cause the player to use the gadgets as a stand-alone weapon (increasing weapon switch time).