Learn English – Source of ‘BB’ in the sense of ‘small, spherical pellet of shot’

Merriam-Webster's Eleventh Collegiate Dictionary (2003) has the following entry for BB:

BB n (1845) 1 : a shot pellet 0.18 inch in diameter for use in a shotgun cartridge 2 : a shot pellet 0.175 inch in diameter for use in an air gun

The two definitions incorporate more-precise measurements than are likely to have been applied back in 1845, when MW says the term was first used in print, and this suggests to me that there may have been a significantly simpler definition (such as "small birdshot") back in the earliest days of the word's use. Also, the 1845 origin date notwithstanding, the first edition of the Collegiate Dictionary to include any entry for BB was the Eighth Collegiate, published in 1973.

The full-size Merriam-Webster's Third International Dictionary (1986) repeats the definitions of BB (which it gives as "bb or beebee … usu cap both Bs") from the Eighth Collegiate and then offers this rather cautious speculation about the term's origin:

[prob. fr. the letter b]

This doesn't strike me as being very informative.

Stuart Miller, Concise Dictionary of Acronyms and Initialisms (1988) has this brief entry for BB:

BB Shot pellet

The Compact Edition of the Oxford English Dictionary (1971)—the edition with the magnifying glass—has no entry for BB at all. The Online Etymology Dictionary doesn't cover the term either.

The closest thing to an explanation that I've found so far is in Wikipedia's entry for "BB gun", which states:

The term "BB" originated from the nomenclature of the size of steel balls used in a shotgun. BB sized shot was normally 0.180 inches (4.6 mm), but tended to vary considerably in size due to the high tolerances in shotgun shells. The highest size shotgun pellet commonly used was named "OO" or "double ought" and was used for hunting deer and thus called "buckshot."

Around 1900, Daisy, one of the earliest makers of BB guns, changed their BB gun bore diameter to 0.175 inches (4.4 mm), and began to market precision-made lead shot specifically for their BB guns. They called these "round shots", but the "BB" name was already well established, and everyone continued calling the guns "BB guns" and the shot "BB shot" or just "BBs".

This discussion corroborates the Eleventh Collegiate's measurements for pellet diameter, and it suggests a first occurrence date for definition 2 of BB of "around 1900," when Daisy standardized the size of the lead pellets for its BB guns at 0.175 inch in diameter. But it doesn't address the origin of "BB" as a classification name, including whether it originally stood for some longer word or words, nor does it indicate whether BB is older or younger than such other birdshot classifications as FF and TT.

The "Shot" subsection of Wikipedia's entry for "Shotgun" notes that current U.S. standard birdshot sizes include (from largest to smallest) FF, F, TT, T, BBB, BB, B, and 1 through 9. It doesn't offer any insight into how, where, or when these classification names originated. Neither does the "Birdshot" subsection of the Wikipedia entry for "Shotgun shell".

I have three questions about BB in the sense of a shotgun or air-gun pellet:

1. What (if anything) does BB stand for?

2. What was the source and original meaning of the term?

3. When and where did it first appear in written English?