The BB in England
The earliest mention of BB, in the sense of shot of a certain size, that a Google Books search finds is from William Osbaldiston, The British Sportsman, Or, Nobleman, Gentleman, and Farmer's Dictionary (1792):
In order therefore, to shew clearly, at one view, the comparative difference in the garnishing of [patent] shot of different sizes, we here subjoin in a table, which indicates the number of pellets precisely composing an ounce weight of each sort of shot.
No. B. B. [– –] 1 ounce – – 60 [pellets]
[No.] B. [– –] id. – – 67 [pellets]
[No.] 1. [– –] id. – – 86 [pellets]
[No.] 2. [– –] id. – – 109 [pellets]
[No.] 3. [– –] id. – – 160 [pellets]
[No.] 4. [– –] id. – – 200 [pellets]
[No.] 5. [– –] id. – – 256 [pellets]
[No.] 6. [– –] id. – – 444 [pellets]
[No.] 7. [– –] id. – – 530 [pellets]
[No.] 8. [– –] id. – – 600 [pellets]
So already in 1792, at least one hunting guide in England was identifying distinct categories of shot size on the basis of how many pellets of that size weighed an ounce. And already there existed the unexpectedly heterogeneous system of identifying shot size from larger to smaller running from 1 through 8, and then identifying shot sizes larger than No. 1 as No. B and No. B. B.
A slightly older schedule of shot sizes—without the B. and B. B. sizes, but with special complications of its own—appears in a 1789 translation of G.F. Magné de Marolles, An Essay on Shooting, along with an apologetic note about the oddness of the category labeling:
No. 8 [- -] 1 ounce - - - 620 [pellets]
[No.] 7 [- -] id. - - - - 480 [pellets]
[No.] X [- -] id. - - - - 300 [pellets]
[No.] 1 [- -] id. - - - - 220 [pellets]
[No.] 2 [- -] id. - - - - 180 [pellets]
[No.] 3 [- -] id. - - - - 157 [pellets]
[No.] 4 [- -] id. - - - - 105 [pellets]
[No.] 5 [- -] id. - - - - 83 [pellets]
The Reader will observe, that the patent shot has no No. 6, the X being substituted in its place, and that the numbers do not follow each other in the order of progression: the reason of this we cannot assign.
The strangeness of this schedule is twofold. First, it inexplicably replaces the number 6 with an X. And second, it reverses direction partway through the chart, listing No.s 8, 7, and X (6) in order of increasing pellet size, but then assigning No.s 5, 4, 3, 2, and 1 in the opposite order by decreasing pellet size. It’s as though someone had started a countdown from 8 toward 0 but then suddenly replaced that system with a countup from 1 to 5.
An article titled “Observations on Shot,” in The Sporting Magazine (September 1793) repeats Marolles’s numbering system (and the querulous note about it) without revision, right down to the X in place of the 6. The entry for shooting in the 1797 Encyclopædia Britannica, third edition (1797) repeats the same mystifying table and mystified footnote; but the entry for shot in that edition of the encyclopedia sensibly and consistently identifies shot categories ranging from No. 1 through No. 6, “and smaller” and discusses the suitable shot sizes to use for specific game birds:
The sizes of common shot for fowling are from No 1 to 6, and smaller, which is called mustard seed, or dust shot; but No 5 is small enough for any shooting whatsoever. The No 1 may be used for wild geese ; the No 2 for ducks, widgeons, and other water fowl ; the No 3 for pheasants, partridges after the first month, and all the fen fowl ; the No 4 for partridges, woodcocks, &c. ; and the No 5 for all the smaller birds.
Earlier discussions of shot used for fowling had described how to make the shot, but they had not mentioned the use of a classification system of numbered sizes of shot to identify the different sizes of pellets that may be produced.
In a table reporting numbers for both common shot and patent shot, William Nicholson, The British Encyclopedia or Dictionary of the Arts (1809) mentions shot sizes No. 1 through No. 10 in the course of describing appropriate shot to use for different birds, but he devotes most of his attention to No.s 4 through 7, which he says are best for fowling. There is no mention here of categories with letters rather than numbers, but to his credit Nicholson abandons the Marolles schedule in favor of a consistent sizing scheme.
B. Thomas, The Shooter's Guide, or, Sportsman's Companion (1811) presents a schedule of eleven categories, consisting of sizes B.B., B., and 1 through 9. Thomas proudly reports that he personally counted the number of pellets in an ounce of each type of shot, the numbers ranging from 58 pellets for B. B. to 970 pellets for No. 9; however, he doesn’t discuss a suitable target for B. B. shot, though shot of that grade appears to be too large for the bird hunting that he is chiefly interested in.
More changes in labeling appear in Peter Hawker, Instructions to Young Sportsmen in All that Relates to Guns and Shooting (1816), which offers this descriptive schedule of shot options for fowling pieces:
Of average mould shot, one ounce is 16 [pellets]
A .......... 49 [pellets]
BB .......... 58 [pellets]
B .......... 75 [pellets]
No. 1 .......... 82 [pellets]
and so on through No. 9 shot at 970 pellets per ounce. Hawker then notes a manufacturing complication with shot designations:
The shot of different manufacturers varies much in size : for example, an ounce of No. 7, from Messrs. Walker and Maltby, amounts to 341 pellets ; and the same weight, from Mr. Beaumont (late Preston), 398 ditto, &c.: and, in some places, the numbers are reversed.
This last comment seems to harken back to the confusion surrounding Marolles’s schedule from 1789.
Fourteen years later, Hawker, Instructions to Young Sportsmen in All that Relates to Guns and Shooting, sixth edition (1830) reports an increase in the number of patent shot categories from twelve to fourteen:
SCHEDULE OF SHOT,
According to labelled samples, which were sent me from Messrs. Walker, Maltby, and Co., Patent Shot Tower, Lambeth. The firm is now Messrs. Walker, Parker, and Co., as Mr. Maltby is established in the shot-business by himself, at the new round tower on the other side of the bridge.
PATENT DROP SHOT
AA .... 40 [pellets of shot per ounce of lead]
A .... 50
BB .... 58
B .... 75
1 .... 82
and so on through 10 at 1726 pellets per ounce.
Luke Herbert, The Register of Arts, and Journal of Patent Inventions, volume 5 (1831) describes the patent granted to William Watt of Bristol, England, on September 10, 1782, “for making small shot solid throughout, without the imperfections which other shot usually have on their surface.” In his discussion of the patented method, Herbert remarks that shot is divided into twelve categories:
The various sizes of the shot are distinguished by the manufacturers by the Nos. 1 to 12; the largest, No. 1, are called Swan shot; the smallest, No. 12, dust shot; their diameter varying from 1-30th to 1-4th of an inch.
It isn’t clear, however, whether Herbert is saying that the 1–12 system was the one in force in 1782, when Watt obtained his patent for shot-tower shot, or whether he is claiming that that system is the one in force in 1831 (which it evidently is not—at least not in Lambeth, England, at the shot works of Parker & Maltby).
Yet another early mention from England appears in Captain Lacy, The Modern Shooter (1846), who likewise identifies a schedule of “patent drop-shot” offered by Walker & Parker in Lambeth, England—this time across15 categories ranging in size from about 1,700 pellets per ounce for “No. 10” shot to about 32 pellets per ounce for “A A A” shot. In a table that Lacy provides, drop shot is divided into 15 categories. In order by ascending size of each pellet, the categories are 10 through 1, and then B, B B, A, A A, and A A A. This time, the “B B” category is defined as having a yield of about 60 pellets per ounce of lead.
The BB in America
To this point, all of the citations have been to English sources, and we have seen an elaborate classification system emerge with A A A at the large-pellet-size end and 10 at the dust-size end.
But now we pick up our first instance from a U.S. publication. From Frank Forester, “Spring Snipe Shooting, or Three Days at Pine Brook, New Jersey,” in American Turf Register and Sporting Magazine (September 1840):
Off we set without further parley—within five minutes I had bagged Tom’s first, a rare green-headed Drake, and joined Van Dyne, who, with the head and neck of his first bird hanging out of his breeches' pocket, where, in default of game-bag, he had stowed it, was just in the act of pouring a double handful of BB into his Queen Ann's musket. Before he had loaded, we heard a shot across the road, and saw the fifth bird fall to Harry at long distance, while Shot [a dog] was gently mouthing Draw’s second Duck to his unutterable contentment.
This is the same article that JEL cites (in another answer) from 1845 in (it appears) a periodical called Warwick Woodlands, but the original version of the article is five years older and from the United States. Notably, Van Dyne is described here as “pouring a double handful of BB”—that is, of BB-size shot—into his Queen Ann’s musket. This is quite a different sense of BB from the “B-B [bulleted-breech]” percussion cap cartridge that JEL notes was invented in 1845 by Louis Flobert. I strongly suspect that the naming similarity between “BB shot” and “B-B [bulleted-breech] cap” is purely coincidental.
Edward Knight, American Mechanical Dictionary, volume 3, A Description of Tools, Instruments, ... (1876) compares drop shot sizes and labeling conventions at manufacturers in Baltimore and New York. The Baltimore schedule includes 19 pellet sizes, ranging from largest to smallest through the categories of TTTT, TTT, TT, T, BBB, BB, B, and 1 through 12. The New York schedule has nineteen categories, too: FF, F, TT, T, BBB, BB, B, and 1 through 12. The BB size in Baltimore yields 45 pellets per ounce, while the New York BB size yields 50 pellets per ounce. These yields are significantly but not hugely different from the English BB size yields of 84 years earlier (60 pellets per ounce) and 60 years earlier (58 pellets per ounce).
Why is the nomenclature for shot schedules so odd?
I am persuaded that the term BB as applied to spherical pellets of lead or other metal began as a size designation for lead shot in England no later than 1792, and migrated across the Atlantic to the United Sates sometime in the first half of the nineteenth century.
My theory about the odd numbering and lettering schemes for shot is based on the notion that shot was originally separated into sizes by running it through a series of screens or sieves. Nicholas Cox, The Gentleman’s Recreation (1721) gives this very brief summary of this part of manufacturing shot:
When you cast your Shot, take them out of the Water, and dry them over the Fire with a gentle Heat, and be sure to keep them continually stirred that they melt not. When they are dry, you are to separate the great Shot from the small, by the help of Sieves made on Purpose according to their several Sizes.
Obviously if you are planning to run shot through a series of sieves, you’ll want to use the screen with the largest openings first, to catch the largest pellets and let the smaller ones pass through. And as this is the first screen you use, it is natural to refer to the resulting large shot that it filters out “No. 1 shot.” The same goes for the second screen and the No. 2 shot that it filters out, and so on down the list to the lead dust particles that pass through the finest-mesh screen at No. 8 (or later, No. 10).
When companies began attempting to standardize the shot sizes they produced, I imagine that they applied this same simple numbering system to the results of multiple screenings of the shot. But when they began to use larger-mesh screens for the first passes than were used in the original numbered sieving or screening process, the manufacturers found themselves in a bind regarding nomenclature: either they had to renumber all of the shot sizes so that the new No. 1 size shot was now two screen sizes larger than the traditional No. 1shot—and so on for all subsequent shot sizes, or they had to leave the existing shot sizes approximately as they were, and come up with a new naming convention for the larger shot sizes.
I think that they took the latter course by 1792, when we read the first mention of sizes B. and B. B. I have no idea what (if anything) the B stood for, but there is no evidence of a size A (or AA or AAA) in English shot sizing systems until 1816, when A shows up as the next size larger than the previous largest size, BB. Because A appeared somewhat later than B and BB, I suspect that B and BB were not being used as simple A-B-C gradations for larger size shot from the outset. Perhaps at first B stood for nothing more complex than “Bigger” and BB for “Bigger Bigger.”
In any event, the English system combining the numbers and the B and BB extensions (but not the A extension) made the jump to the United States, where the inevitable high-end increases in pellet size acquired different letters from the ones that emerged in England during the later 1800s.
The BB owes its preeminence among the shot categories to the emergence in the late 1800s of the BB gun, a single-shot long-barreled gun that fires a BB-size pellet. Although the Daisy company is famous for gaining a huge following for its BB guns, around 1900, and for slightly altering the size of the standardized pellets used in them, less uniform BB guns were already widely used by the early 1890s. This is evident from the earliest match that a Google Books search finds for the term “BB gun”: it’s from a comment in American Law Review (July–August 1892 )discussing state court rulings that a parent who gives a child of nine, ten, or eleven a BB gun and the child or a friend of the child later puts out someone's eye with it is not guilty of negligence for having entrusted the gun to the child.
I haven’t been able to find a history of shot nomenclature that spells out precisely how the parallel systems evolved in England and the United States, but it appears that the earliest schedules of differentiated sizes of shot (from the late 1700s) began with an all-numerical system based on the order of the sieves or screens used to filter out pellets of roughly equal size, starting, for obvious reasons, with wide mesh sizes and growing progressively finer.
The BB specification appears to have been a later edition (from the early 1800s) that arose when shot makers introduced new screens at the upper end of the process but chose not to renumber the existing screens and screen-based shot sizes. This led to the use of letters for the next-larger sizes, starting with B, and BB.
The English shot schedules arrived in the United States after the B and BB sizes had been added to the basic numbering system but before the still-larger A, AA, and AAA sizes came into use. From that point onward the English and U.S. systems went their separate ways, and in the U.S. sources I consulted there is no indication that A, AA, and AAA were ever adopted, the larger sizes being instead assigned to T, TT, F, FF, etc.
A final complication worth noting is the emergence of so-called “B-B [bulleted-breech]” percussion cap cartridge in the United States in the mid-1800s. These cartridges seem to me to be etymologically unrelated to BB shot.