The Dutchman states: "Don't ask me why the original rimfire case is 42mm long and the civilian centerfire case is 44mm long.
I don't know!"
Well, I do know; originally both the Swedish and Norwegian military rifles chambered the 42mm. cartridge. After a few years the Norwegian Army decided to switch to a slightly longer and heavier bullet. In order to retain the same powder charge they lengthened the chambers and the brass cases by 2mm. The Swedish Army however did not adopt the new bullet and longer cartridge.
That's why most original milspec Swedish Rolling Blocks have 42mm chambers, while most all Norwegian military Rolling Blocks and most civilian/sporterized Scandinavian Rolling Blocks have 44mm chambers.
I have had .50-70 Starline brass converted to fit a sporterized center-fire Swedish Rolling Block with a 42mm chamber.
It involved cutting and trimming the length in order to fit the chamber.
I had to have the diameter of the rim reduced in a lathe in order to chamber.
Finally I had to run the cases all the way up into a .56-50 Spencer sizing-die in order to reduce the web-diameter sufficiently to allow the cases to chamber. The Spencer-die is tighter than the corresponding .50-70 die, but you still will need to ram them all the way into the die without using the shellholder, and then use a wooden dowel and mallet to knock the sized cases out of the die. It's a lot of hassle.
I recommend instead using the .50 Alaskan cases by Starline, which in most guns only need to be cut to the correct length in order to chamber.