From Shotgun Life 2013 article:
As to allowable diameter variation tolerance, SAAMI voluntary performance standards state that game shot (actual shot sizes falling into this category undefined) are allowed a nominal diameter tolerance of ± 0.010 inch, target shot (again actual sizes undefined) are allowed a diameter tolerance of ± 0.005 inch and buckshot (shot sizes defined as No. 4 Buck through No. 000 Buck) are allowed a nominal diameter tolerance of ± 0.015 inch. So a pellet made in the U.S. in the game shot size range could vary a full pellet size designation either way (remember: the variation tolerance is ± 0.010 inch) and still be called that pellet size. Example: according to SAAMI a group of pellets could vary in diameter from as large as No. 4 to as small as No. 6 and still be honorably represented on a bag of loose shot sold for reloading in the U.S. or loaded into American factory ammunition as No. 5 shot. Things get tighter for target shot (typically thought of in the U.S. as being No. 7 pellets or smaller), as the allowable diameter variation tolerance is only a half size (± 0.005 inch). But hey, when you think about it, you want to be pretty discriminating about so-called No. 7½ or 8½ shot for according to SAAMI voluntary performance standards for target shot, a group of pellets could vary from as large as No. 7 to as small as No. 8 and still be designated as a No. 7½ by the manufacturer. In like manner, so-called No. 8½ shot could be as large as No. 8 to as small as No. 9 and still be designated a No. 8½.
