I have loaded alot of the older style wads in the 2.5in 12ga for short chamber English doubles, and would recomend the hulls that are straight wall, not the new std tapered hulls, for use with what you are considering. PB powder is a good choice for some very good low pressure field velocity loads.
23.0 of PB, 209 win or fed primer, one .135 nitro card, 2ea 3/8in fiber wads, 1 1/8oz shot, in a Fed paper or straight wall plastic hull. you can adjust the wad column to attain the proper crimp depth depending on the case used. this should give 1150fps at a sedate 7400LUP.
I have used this load in a VERY EXPENSIVE double gun with full confidence, I sure wouldnt want to be reponsible for pranging Winston Churchills grouse gun (documented).
The first edition of the Lyman shotshell handbook is a good one to find, and full of older load data thats still relevant
Hope this helps you to reach your goal.
P.S. you might have access to the Canuck paper hulls ,they are good too.
