IMR stands for Improved Military Rifle powder. 4756 & 7625 were classified by Duont as Sporting Rifle powders, thus SR. I believe that PB was classified as a Shotgun Powder. In any case the PB designation related to its mode of manufacture & stood for Porous Base. It is still a hard gelatinized powder, not a fibrous powder as was the old bulk EC, but has itty-bitty holes in it.
It should give much cleaner & more efficient burning under low pressures & cool temps than either of those SR powders. PB is a faster powder than 7625, I would definitely cut the charge wt from a 7625 load.
The 10ga bore has 13% more capacity than the 12ga. Personally I would feel much safer to find a 12ga load using PB with 1 1/8oz in the desired pressure range & then loading 13% more powder with the 1Ľoz in the 10ga, than in trying to work with 7625 loads.
Green Dot is a very efficient powder in a similar burn range to PB. It normally uses a lighter charge wt & also usually can be found for lesser $ per lb.
