|The following is based on Van Buren and has not been confirmed by canon sources.|
BBs are a type of ammunition that was to appear in Van Buren, the canceled Fallout 3 by Black Isle Studios. It is also featured in the Van Buren tech demo game files.
There is no mention of any weapon chambered for it, either in the tech demo or the design documents.
While it doesn't appear in the Van Buren tech demo, it is still present in the game files and can be spawned with the console command CreateEntity AmmoBB.amo.