John;
An old Alliant (Formerly Hercules) handloaders manual has a chart in it showing the length of shot olum for 1 oz of shot in gauges from 10ga down through the .410 bore. They did not state either the size of shot or actual alloy, either of which would affect exact results. I highly suspect it was done with ordinary lead chilled shot of a smaller size as ordinarily used in skeet/trap shooting. Per this chart 1 oz shot has a column length of .690" in the 12 ga bore of .729" so under square by .95:1. 1 1/8oz would thus have a length or .776" or 1.065:1 over square. 1 1/16oz is as close as one is going to get to square in the 12ga. 1Ľoz would have a length of .8625" or a ratio of 1.18:1. I have personally never witnessed any problem with using 1Ľoz of shot in a 12ga as regards its patterning ability.
However lets switch now to the 20ga. If you load that same column length of .8625 you will have 7/8 oz of shot.The 20ga has always as far as I am aware been noted for its hreat ability to handle 7/8os of shot. BUT the ratio of length to diameter in the smaller .615" bore of the 20ga is now 1.4:1.
My question now is if we load the 12ga with Ľoz of British #6's & the 20ga with 7/8 oz of british #6's the bottom layer of shot in both cases has exactly the same number of shot stacked on top of them. What difference does it make that one load has a ratio of 1.18:1 & the other is 1.4:1. In fact if you load each gauge with a shot load proportionate to the square of their diameters each one will have the same column length, but the ratio goes up as the gauge goes down. If one checks loads real close it will be found this common length of column is much closer to the loads traditionally loaded in the different gauges of shotguns rather than loading to a common ratio relative to their diameters.
