I just did up a 6mm CM for a guy.
Very easy cartridge to do. 243 with the bullets seated long, it fireforms and then you stuff it full of a slow powder like H1000. You'll get between 3500-4000 rounds and still keep the velocity for slingin the big 105-115 grain 6mm bullets out to 1K. A BR will certainly work in your action but unless it's going to be a single shot, getting it to feed in a box magazine is going to be a challenge. Historically they've never been easy to do, although it is possible.
Reamer is in stock and I'm pretty sure I have 8 twist barrels as well.
Target rifles for International Palma, NRA Highpower, Benchrest, Silhouette, and Olympic smallbore/NRA prone, and 300m are my specialty.