I have been working up loads for a Swedish rolling block in 12.7 x 44R. It is a sporter with a 33" barrel. Fairly light, 'caus a that big hole down the middle! It does have a wide military butt plate.
I use Starline .50-70 brass, shortened slightly, and the rim narrowed to fit. Dies are regular Lyman. Bullet is the 350 gr Lyman 515139, lubed with SPG. (Not lubrisized, just rubbed the lube into the grooves by hand.)
First test, 70 gr. 2F Goex + card wads; 1300fps, good group @ 50x but low & to the left. Sights are barleycorn, with a very fine "V" notch in both a standing leaf and a second fold-up leaf.
second test; 75 gr. 2F + a single card wad; 1350 fps. Grouped 3" - 4" above poa @ 100x with the second leaf raised. Group widened because I didn't clean between shots. NOTE: COTW3d says that the factory load is 75 gr of 1F with a 350 gr. bullet
third test; 80 gr. 2F without wads. 1430 fps. cleaned between shots. grouped better, but lower than before. This time recoil from the bench started to smart, even with the wide buttplate, and hand-held solidly back into the shoulder.
I am loading with a view to shooting a moose with this old girl made in 1870. Next I will drop back to 75 gr. of 2F Goex and try to refine both the load, and myself. I reply here as the 12.7 x 44R seems to be the direct cousin of the .50-70, both being derived (My opinion) from the .50-70 rimfire.