I would not be putting jacketed .510" bullets down that bore. Grease groove or paper patch all the way, smaller than .510". Cheaper than Woodleighs that way, too. At a guess, with a nominal bore of .490"ish, plus 2x a groove depth of a nominal .006"ish, something a tad over .500" will be roughly what you'll need; but it'll also depend on the rifling. Is it Henry? Henry rifling can have some taper in the bore. Sometimes. Slug and chamber/throat cast will be important here.