This has probably occurred to others, but in fiddling with various .50-70 rounds and noting they are not drop-ins in an original 12.7X44R Swedish roling block sporter, I lit upon a .56-50 round loaded with .512 bullet in Starline brass for my Spencer repro, and it was a very nice fit. Of course, there is an issue of case length and throat in the original RB chamber. I will test a few of these when I get a chance, however, and report on performance. I suspect they will work just fine  as a reduced load.