The rail was implicitly used in Battlefield 2, Battlefield 2142, and Battlefield: Bad Company 2 for their respective assault rifle attachments.
"Used to attach underslung grenade launchers and shotguns. Without it, these weapons will be equipped stand-alone."
Default (Assault Rifles)
Available for most assault rifles, the Underslung Rail allows either the M320/GP-30 or M26 MASS to be mounted to the rifle instead of being used standalone, decreasing the time needed to switch between the two weapons. Any optics mounted to the rifle can also be used with the underslung weapon, though the M26 benefits more from this property. The rail is not available on any of the bullpup style firearms except the AUG A3.
While using the underslung weapon, the soldier carries the weapon in a manner suggesting its use. M320/GP-30 users will tilt their weapon back, while M26 MASS users will flip out the weapon's left-hand bolt.
If a Medkit is selected, the rail will have no effect whatsoever. Also, certain combinations of weapons cannot be mounted together:
"Used to attach the M320 or GP30 Grenade Launcher in the underslung position for faster application of 40mm grenades."
• Default underbarrel option