IMR stands for Improved Military Rifle powder. 4756 & 7625 were classified by Dupont 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.
I shot many rounds of trap last summer through a twist barrel Parker using Federal hull trimmed to 2 7/8", Win 209 primer, 22 grains of green dot, SP10 wad, enough 16ga filler wad to make a good crimp (about 1/2"), 1 1/8 ounce of lead shot, star crimp. This is a very comfortable and effective load but it is of my own design and has not been pressure tested by anyone I know. My logic is there are similiar 12ga loads of that amount of GD and shot and the 10 gauge is going to have less workign pressure due to bore size. Works great for me, use at your own risk.
