SAAMI probably dictates the O.D. of shot sizes today but in the early days of shotgun shooting each manufacturer had it's own shot size and they were similarly numbered, e.g. 4,5,6,7,7 1/2, 8,9,10,11, but O.D.s sometimes varied. Not to say that they weren't close or exactly the same between manufacturers in most sizes but some varied considerably. Some even had shot so fine it was called "mist" or "dust" shot. A steady diet of smallish birds taken with such shot certainly must have produced a plethora of "village idiots"
