Note the major factors in pressure is the wt to be lifted per SqIn vs the speed with which it is lifted IE charge wt & burn speed. Most 12ga low pressure loads are based around an ounce of shot. bore area of a 20ga in ratio to a 12ga is 71% so a 3/4oz load is absolute max when searching for equivelent LP loads to the 12ga. You can get equally low pressures in the 12ga using 1¼oz as in the 20ga with 7/8oz. In Black Powder days 3/4oz was standard for the 20ga & 7/8oz was considered the "Heavy". The reason 10 & 12 ga guns were so popular a century ago was they carried the 1¼ & 1 1/8oz loads so useful to hunters. The 16ga with it's 1oz load was considered abiout perfect for upland birds by many, particularly in the Grouse woods & Bobwhite fields of the South.
