There are several factors which make a slower powder more effecient in a smaller gauge as 16/20 than in the 12gauge. First & generally speaking you are using the same primer to light a smaller charge in a smaller space, thus better ignition. Second & this also isa generalization, but usually the smaller gauge will be loaded heavier in proportion & will also have more bore friction even for the same proportionate load. In proportion to a 1oz load in 12ga a 16 would have a 13/16oz load & a 20ga just over 11/16 (23/32 actually). These gauges are seldom loaded that light but the 12ga is quite often used with a 1oz load. Keep in mind though the 7625 load which let me down was a 1¼oz one in 12ga, though it was loaded in the old Rem SP hull with paper base wad & a mild primer (CCI 157). "IF" you feel you just have to go to those extremely low pressure loads with a slow powder use the lowest pressure you can with a hot primer & hot case (AA etc). Personally any gun I did not trust with a 7500 psi load when I went afield I would leave on the rack at the house. 1oz of shot @ 1200 fps is going to stress the action, stock etc etc about the same regardless of max chamber pressure. The max chamber pressure is primarily going to affect the Chamber walls. Except for very early guns built before introduction of smokeless, some of which are fairly light in the breeches, the chamber is about the strongest part of a shotgun. Those on which it is not I personally reserve for only light loads of BP.
