Geno:
Sorry for this late reply, been busy lately. I take it you are intending to regulate the rifle in the normal way, perhaps with a wedge at the muzzles, when re-laying the ribs. You will need to choose a load to regulate with.
Notwithstanding the strength of an Englishman's shoulder, a .577 Express rifle weighing around 9 pounds will not be comfortable to shoot with juiced-up loads. I would strongly suggest sticking to the original factory loads for that rifle, ie 520gr lead bullet over 160gr Fg for 1725 fps (Eley catalogue, 1910-11), which should also regulate perfectly with a 570gr jacketed bullet over the correct amount of medium-burning smokeless (~75gr)to achieve 1650 fps (Nobel catalogue, 1925).
For a good hunting load, I might also suggest trying the Woodleigh 650-grainer (Cat No.5 for BP) at 1650 fps with your equivalent of Varget/IMR4895/Reloader15: I shoot that load in my Greener conversion as you know. It weighs 10.5 lbs and is very comfortable to shoot, but recoil would be stout in a 9-pound gun. Anything more would be silly IMHO.
