According to a table from Burrard the old standard British load for the 2" 12 was 1 1/8oz, which was reduced by Law I believe during WWI to 1 1/16oz. As far as I know the 1oz was generally considered a "High Velocity" load in the 2" 12 but never Standard Per Se. Standard for the 16ga 2" was 7/8oz & for the 20 3/4oz.
Guess what 1 1/16oz in 12, 7/8oz in 16 & 3/4oz in 20 will have almost identical shot column lengths which means they have almost equal density of loading. They will thus fit into the same length shell with about the same amount of wadding etc. Wonder if that was co-incidental. If loaded to identical velocities with the same powder each step down in gage size would show slightly more pressure due to a higher percentage of the shot making bore contact, thus more friction proportionally.
