Based on data from older Hercules/Alliant handloader's guides, which gave the length of an ounce of shot in bores of the various nominal sizes.
Taking 3/4 oz as the means then the following are the nominal lengths of the shot column length in the un-protected bores, using regular chilled lea shot of small size.
.410/.410" = 1.63125"
28/.550" = .9075"
20/.615" = .726"
16/.662" = .62775"
12/.729" = .5175"
Two things worthy of note;
#1; the length of the column in the barrel has a very small effect on shot string after the shot eves the muzzle.
#2; You can bet the farm it has a much more profound effect on the central core thickening of the pattern, with the loner columns showing a far greater tendency to high core thickening with a much less uniform pattern throughout.