I have had a 12.7x44 for 20 yrs or so and have only ever seen one cartridge, that was in a large cartridge collection at the Cal easter gun show...they are scarce in this country.
I have tried fire-forming .348 factory brass (some authors that I cant remember their names encouraged this at one time) but my success was dismal to say the least. some cases would split down the side and others, because the base is a bit smaller on a .348, would bulge on one side & not the other so the primer pocket was off-centered enough that the fp wouldn't hit enough of it to fire.
Now for the trials & tribulations of using 50-70 in a 12.7. If you don't already know the rim on the 12.7 is somewhat smaller than the rims on factory 50-70. You have two options remove the barrel from the action, chuck it up in a lathe and open up the rim inlet to suit. This might sound simple but two slab sides & steep taper of the original barrels requires a jig to be made that will hold the barrel straight and true in the chuck when the cut is made ( it took me 3 times a s long to make the jig than it did to make the cut). The other , more simple "fix" is to have the rims cut down on the brass you intend to use.
Another factor to take into consideration is that all 50-70 brass is not created equal, some is "drawn", some is "machined" ( the least desirable in my opinion) and all 50 Govt brass isn not the same length although I think all the new-drawn stuff is the 50-70 length. The US govt made 2 different lengths of brass for the .50'sm, a shorter one for the original gatling guns & the trap door carbines ( I have one I my collection) that hold a reported 55 gr of BP. These short cases are scarce as well, Numerich & Dixie Gun works sold them a long time ago but I have seen them at gunshows in used brass grab-bags.
Any of the drawn brass, even the short ones should work for you but every machined brass I have tried to use was either too big or too thick to seat a bullet in and chamber.
Good luck...a little adversity is what makes some of this hobby so rewarding when things finally work for you