BB

 are a type of ammunition in Fallout, Fallout 2 and Fallout 3.

Fallout and Fallout 2
BB's are going to be a very useless ammo type to most characters. They're heavy, they're rare, and 2/3rds of the guns that chamber them are useless compared to the starting guns. However, if you run across Bob's Pre-owned Car Mart, or revive frozen soldiers in Sierra Army Depot, you might find the Red Ryder LE BB Gun, which makes BB's deadly little buggers.

Fallout 3
BB's are a small guns ammunition found in Fallout 3. They are used in only one weapon, the BB Gun.

Locations

 * On top of VAPL-58 Power Station: a gun and 3 ammo containers.
 * NW from Scrapyard there is a power station. The first power pylon south of it has a gun and 2 ammo containers.
 * 300 rounds on the assembly line of Mothership Zeta Cargo Hold section.
 * Over 100 to the right on a shelf next to 2 BB guns just before you enter Haley's Hardware (Point Lookout)