There is a great deal of discussion when it come to the optimal bullet diameter for the 577/450 Martini cartridge and it it's Henry rifling.
Here is a the official description of the British Military Henry rifling:
Using only to figures given in this official description what should the optimal bullet diameter of a cast lead slug used in this bore be?As issued, Henry's barrel had a nominal bore of 0.450-inch, obtained by allowing an 0.449-inch plug to run, whilst rejecting one of 0.451-inch. This bore was slightly enlarged at the breech end, so that an 0.451-inch plug would run in four inches from the breech, and one of 0.450-inch, eight inches. From this point to the muzzle the bore was cylindrical. The grooves in the tapering section were 0.009-inch deep to within one inch of the true bore, and from there to the muzzle 0.007-inch deep, giving a relieved bore of eight inches, with progressive grooves for seven inches. There were seven grooves of a right hand uniform twist, with one turn in 22 inches. The chamber was cut in the barrel, and was coned 0.018-inch to ease extraction.