Od Hercules/Alliant handloaders guides used to state how long a column 1 oz of shot would be in the various gauges. Neither shot size nor alloy was stated. Presumably this would have been ordinary "Chilled" shot of a size suitable for clay targets. Non shot cup wads would be used to obtain these lengths which are based on a volume of .288 CuIn per ounce.
10ga = .610"
12ga = .690"
16ga = .837"
20ga = .970"
28ga = 1.212"
.410 = 2.181"
Larger or harder as well as shot cups will of course result in a slight lengthening of the column in the bore.
Does this result in a longer shot column in the air, don't know, has been debated for years.
When firing the same load from different bore diameters though this length to diameter ratio is why the smaller bores require a slower burning powder to keep pressures from going out the top. 1oz in a 12ga is quite nicely handled with Red Dot, but don't try it in a 28 with an oz of shot. To attain similar velocities the smaller bores will normally require heavier charges of the slower powder than will the larger bore using a fast powder.
