I have a RB in 12.17 that I would like to have converted to .50-70. The breech block also has a tiny amount of play, I would like to fix that. Can anyone suggest a good smith with experience in rolling blocks? I'm located in BC. 
If my memory is correct, all that is needed is to run a chambering reamer in; your metric bore should be around .513, should it not? Also some of the guns were rimfire and some were centerfire, so you may have to modify the breach block to centerfire. Final point is that if the slop is only minor, that may not be a problem; if the gun is chamber so tight that the cartridge base pushes on the breach block, the breach can slow down the hammer so that it does not fire on the first fall.
cheers mooncoon
Have you checked if a 50-70 cartridge can go in? Some of these old swedis rifle have very loose chamber. Personnaly, i use cutted-down 50 Alaskan and it fit well; easy to do and less expensive than a reamer.
Check this site: dutchman.rebooty dot com/rb.html ...
I use fireformed 348 Win. brass in mine. Just cut it down to the right length and load with 50-70 dies. You need to expand the neck the first time to get the bullet in but after that you're away.
No chance. Swede chambers vary widely, I had one that would work with .348 cases, but this one, no. Waaay too small for the chamber, won't even engage the extractor.



























