The "True" definition of a Square Load for a shotgun is one in which the length of the shot column in the bore is equal to the bore diameter. Approximate loads which give this to the nearest 1/16oz are;
4ga = 3 3/16oz
8ga = 1 11/16oz
10ga = 1oz
12ga = 1 1/16oz
16ga = 13/16oz
20ga = 5/8oz
28ga = 7/16oz
.410 = 3/16oz
Using the 12ga as standard then the proportionate weight of a shot charge which would give an identical length of column in each gauge, again to the nearest 1/16oz is;
4ga = 2 1/16
8ga = 1 5/16
10ga = 1 1/8
12ga = 1
16ga = 15/16
20ga = 11/16
28ga = 9/16
.410 = 5/16
In loking over these loads it is readily apparent that traditionally over the years the gauges smaller than the 12 have been more often loaded with a column length similar to that of the 12 rather than with a "Square load" = to their own diameter.
The oz load in the .410 has a column length about = to 1 9/16oz in the 12 while the 3/4oz = a 2 3/8oz load in the 12 for length.
