Note that in proportion to bore capacity the ½oz .410 load has an equivelent load density to a 1 9/16oz load in a 12ga. A 3/8oz .410 charge has a LD equal to a 1 3/16oz 12ga laod. Thus changing the .410 by 1/8oz is equivelent to changing the 12ga by 3/8oz. We would certainly expect to change powders if making a 3/8oz shot change in a 12ga. I did not & Do not recommend anyone load untested loads. Also as noted in the days that data was given the .410 cases were loaded with milder primers, which to the best of my knowledge do not exist today. In the days though when a 3/8oz charge was considered about Max in the .410 it was loaded with ordinary shotgun powders. The speciality powders, which in reality are fast rifle powders (2400 was standard for the .22 Hornet) were introduced to the .410 for the purpose of "Magnumization". Note that in proportion to all regular gauges even the ½oz load in the .410 is of magnum proportion. "IF" pressure tested data were available Unique should certainly not be out of range for a max of 3/8oz in the .410.
Again, though I am not recommending pivate load experimentation without pressure testing equipment, just mentioned it for historical reference.
