Greener (1910 Edition reprint) lists two British standards and ten American ones.
The first British one is for Walker, Parker, Ltd., which lists SSG as 15 pellets to the ounce. Given a density for pure lead of .4096 lb/in3, this works out to a diameter of .269". This entry (along with SG and SSSG) is asterisked and footnoted as one of the "Walker, Parker, & Co. London Sizes"
The other British one is for the Abbey Improved Shot Co., which lists SSG as 11 pellets to the ounce. This works out to a diameter of .298", which would correspond to Special SG from the third link I found.
One of the "American" standards is that of the Montreal Rolling Mill Company, which gives SSG as .32" diameter, or 10 per ounce. This one seems to explain the Imperial 0 buck/SSG labeling. None of the other American standards lists sizes any size bigger than 22 to the ounce.
At this point, I think that .27" is the correct diameter for SSG using whatever UK standard is in common use now and that Imperial used a different standard from it.
The first British one is for Walker, Parker, Ltd., which lists SSG as 15 pellets to the ounce. Given a density for pure lead of .4096 lb/in3, this works out to a diameter of .269". This entry (along with SG and SSSG) is asterisked and footnoted as one of the "Walker, Parker, & Co. London Sizes"
The other British one is for the Abbey Improved Shot Co., which lists SSG as 11 pellets to the ounce. This works out to a diameter of .298", which would correspond to Special SG from the third link I found.
One of the "American" standards is that of the Montreal Rolling Mill Company, which gives SSG as .32" diameter, or 10 per ounce. This one seems to explain the Imperial 0 buck/SSG labeling. None of the other American standards lists sizes any size bigger than 22 to the ounce.
At this point, I think that .27" is the correct diameter for SSG using whatever UK standard is in common use now and that Imperial used a different standard from it.





















































