That method of loading should be simply referred to as an Equal Volume loading. Then there is the round ball equivalent loading & finally the Square Load. The square load is as stated one in which the length of the shot column in the bore is equal to its diameter. Thus a cross sectional view of the charge taken longitudinal will appear square in a drawing. That's all there is to it. I see no evidence that ammo loaders have ever followed the Square principal. What they followed was a load in which the column was very close to the same in all the gauges which "Fit" the burning rate of available powders. About as close to square as you will get is 1 1/16oz in 12ga & 1Ľoz in 10.All smaller gauges were normally loaded "Heavier" than square. Stepping up to the 8 gauge a square load would be bit under 1 5/8oz. This gauge was normally heavily built, had long barrels to burn a coarse powder & was often loaded with quite heavy loads.
the square load is for the most part & accidental occurrence & generally will only fall within light loads in the 12 & 10.
Of far more importance is a "Balanced" load which matches shot weight with powder burn rate. Shot loads took a turn upward in the 1920's with the introduction of progressive burning powders. Coarse Black Powder was slow enough burning that heavier shot loads would have been possible but one was limited by case capacity in the "Breech Loaders" . I do recall in W Greener's book The Gun, 1834 he mentioned loading a 5/8" bore muzzleloader with 1˝ oz shot. 5/8" would of course be a slightly over bored 20 gauge.
Bottom Line is the best way to treat the "Square Load" is to forget it ever existed, it would appear to be primarily the figment of someone's imagination & never truly existed to begin with.
