I’ve no load data for that rifle but I did own an XR 45 some time ago. It was warranted safe up to 150 grains of Black Powder (or the substitutes). I shot longish .451 sized bullets intended for the Rigby target rifle and it was very accurate with the mid range charges 70 to 100 grains of 2 and 3f Swiss. That’s a relatively light rifle and recoil was notable above 80 grains or so. If it were me, I’d start at 70 grains volume of the RS and stop around 90 grains volume and my chosen projectile. Then I’d try the same with Pyrodex P. Once you find a promising load you can experiment with overpowder wads, different sizes of bullets, etc.
. Powerbelts have never impressed me from any rifle, properly lubed lead bullets work better IMO. PM me and I’ll send you a few No Excuses 285 grain flat nose bullets sized .451 they should work well from your rifle.