There are multiple issues with bullet size and composition for the 577/450. First off, the unique Henry rifling removes a lot of metal, leaving the minor diameter about a true .45, but the major diameter .466 - .468. In addition they have a long throat, which of course is even wider. The bottomline for Brit military issue MH rifles, a bullet diameter of .468 to .470 (the typical .002 over major diameter) will work best. Bullets can of course be patched up to that and indeed the original was paper patched. The large size of the 577/450 case is a problem for reduced loads, whether smokeless or BP. The case is already grossly oversized for use of smokeless to BP pressure, so reduced loads make it all the more so. Use of BP down to 70 grains is readily done with a fiber filler, such as cotton, wool or kapok. Since recoil is a function of both powder and bullet weight, patching up the dirt common .457 405 grain bullets and use of 70 grains of BP greatly reduces felt recoil and maintains a velocity sufficiant for reasonable accuracy. It must be said, however, that the vast majority of shooters have found their most accurate loads are in the range of the military load. After all, that is what the rifle was developed to shoot.
The free recoil of the MH has been overstated. It is no greater than the vast majority of military rifles (and a lot less than some) up through WWII. However it was designed to be shot from standing or kneeling position where the body moves with the rifle, and not a bench.....which of course many if not most shooters start from. Stock design did not consider modern ergonomics at the time and we must work with what they gave us. If bench shooting, a little padding goes a long way in reducing the bite of the buttplate.