As to a "Square Load" for many years Hercules/Alliant in their reloader's guide gave a chart of the length of the shot column in each gauge for an ounce of shot. Presumably, this was for ordinary chilled lead of a size around 7-8. Lengths were for 10 gauge .610"; 12 -.690", 16 -..837"; 20 - .968"; 28 1.210" & .410 -2.175". As a square load is defined as one in which the column length in the bore is equal to the bore diameter then simply divide the bore diam by this length & it will give you the Square load for that gauge.
Personally I have never placed any stock in the Sq load theory. I have fired 1 oz loads from 12, 16 & 20 gauge guns & the only difference I could ever tell was if one gun fit better I could shoot it better. The actual gauge had very little to do with this effect.
