In an H&R, shooting factory loads are not too comfortable from many of those that have had them, they're equivalent to healthy levergun 45-70 loads, no one makes trapdoor factory loads for the 450M which are the most popular in H&R 45-70s. H&R made them in an Ultra version once, then continued to offer it as an accessory barrel for years, most likely until the existing run of barrels dried up and then it was discontinued, to my knowledge they never were made as an extractor model, if they were, it would indicate that more than the original run(s) were made. For handloaders, there's no difference, they're ballistic twins loaded to levergun pressures, I know of no ruger/modern rifle data for the 450M, the 45-70 has more case capacity, 79gr water compared to 74gr for the 450M.
Tim
