BB A term with two distinct and separate meanings, the most common being the size of air rifle barrels or air rifle spherical steel shot - .175" in diameter. So widely used in this respect that 'BB gun' has become a generic term for all types of spring- and air-operated guns which use this type and size ammunition.
In reference to shotshells, a specific size of lead shot - .181" in diameter. Contrary to popular belief, BB shot size is not identical to BB ammunition size.
Referece: Firearms Encyclopedia, by George C. Nonte, Jr.