The .50 ctg. bullet will work just fine in a rifle.
For the deeper rifled barrels such as I had on one fast twist Bauska barrel, I pushed the .50 cal bullets through a piece of the same barrel, heavily coned (crowned) on one end to allow imprinting the rifling lands into the bullets. The bullets were then held in a piece of "Flambeau" loading block that had the tops (bottoms) cut out to allow the bullet noses to poke through. Because the bullets had the rifling imprinted on them, they had to be 'mechanically' lined up to engage at the muzzle when loaded. This worked perfectly. With some bullet designs, and especially with shallow rifling, the 'blow' of the powder charge will 'slug' up the bullet to fill the rifling even though the bullet is riding on the lands when loaded.
For designs not mechanically loaded (no rifling imprints) the bullet, to ensure good accuracy - nay good but excellent accuracy, I found the bullet had to be .001" to .002" larger than the bore, thus it 'engraved' slightly upon loading and sluged up to fill just fine. If it was too loose, as in a minnie bullet, accuracy was poorer and fouling excessive.
A shortened version of the .50/70 bullet would work better - maybe a 350gr. to 380gr. design, but with the square grease grooves that hold more lube. For lube, use one of the better black powder lubes - Lyman's Black Powder Gold, SPG or perhaps 60/40 mix of beeswax/olive oil (or neetsfoot oil) I've use vaseline with the beeswax in the past and had excellent results, but olive oil or neetsfoot oil 'should' be better yet. By good shooting I mean that shots 2 through 15 or 16 should be perfect with no wiping and easy loading. You can try a lubed ox-yoke-type wad down on the powder as it may help, or not as the case may be. It may not be necessary - I never used wads.
For mechanically fitted bullets, you can easily make up a die to fit a Lyman or RCBS lube sizer from a piece of the barrel you are going to use. Cut the generous crown on this new insert, and use the handle to push the already hand lubed bullets through. The cake-cutter/lube pan method can be used to lube bullets. As noted, the yellow Flambeau handloading ctg. blocks can be used for bullet blocks for .50 cals. The bullets fit perfectly. Merely cut the bottoms out with a dremel tool and carbide cutter shaft - or a drill in a drill press.
Hope this helps.