I have recently obtained two Swedish rollers in 12.7X44R. I've been making loads for them using slightly modified Starline 50-70 brass. I ordered 12.7X44R from Buffalo, but they arrived as formed .348 brass. The rim was smaller and rounded. Not as good as my own made from 50-70, so I sent them back. one of my guns would not eject the brass well.
The guns have been a hoot to shoot!
One of mine is a sporting rifle, built that way by Husqvarna, the other is a military gun that has been sporterized. They have chambers that are different from one another and I keep the loads/brass seperate and treat them as slightly different calibers.
I have been loading with FFg black powder, and would reccomend it. It has been powerful and accurate. I use 50-70 Lyman dies. I ordered 12.7X44 dies (and paid twice as much). They were returned with the brass, as not suitable.
I'm just learning BPC reloading, but even my first attempts are satisfactory, showing group potontial greater than the open barrel sights can produce.
This has been a most enjoyable project and I wish you success w/ yours.