Interestingly if you put 1 1/16oz in a 12ga, 7/8oz in 16ga & 3/4oz in a 20 ga they will all have a shot column length of around .730" in an "On size" bore with no shot collars involved, just the shot in a bare bore. With a proportionate powder charge all would give similar ballistics @ similar pressures using the same powder. Due to a lesser percentage of the shot in bore contact the 12 pressure would be a bit lower & this was generally taken advantage of by upping to 1 1/8oz.
Stepping up just a bit it will be found that 7/8oz in the 20, 1oz in the 16 & 1 3/16+oz in the 12 again have a near equal column length or load density. Again also the 12 will be found loaded slightly heavier @ 1Ľoz.
Most of the old loads were developed around existing powder availability, they didn't just happen Haphazardly.
