Dave;
Checked my files (courtesy of the 16ga. society) and could'nt find any info for a 1oz. load using 20/28 but as Mark suggested contact Armbrust if you really need 1oz. That being said, I find 3/4 or 7/8 extremely effective on targets and game, and very nice on the shoulder/gun. I follow the 16ga. society forum and am amazed at the loads people want to stuff down a 16ga. bore. (1 1/8 and even 1 1/4)
Just buy a 12 if you need that much. As Americans we feel that bigger is better. As it turns out, the older I get the more I enjoy lighter guns and light loads.