Go to 6mmbr and click on gunsmiths in the left colomn. Go to Mike Bryant and give him a call. He converts the standard setup to shoot the 6br. He is the man if you want to have it done. Great smith.
Im a gunsmithing student in Colorado and a fellow student just finished building his custom rem 700, which he did in 6mmBR. It is a short action and feeds well. If using the longer heavier bullets you may want to go with a long action. Whether you go with a long action or a short action you will probably need to have a gunsmith modify the follower. good luck!
It can be done, I used to have one.Bought one built by Ken Johnson at K&P gun co. It was a short barrel, 18 in. and shot great. Wacked many a squirrel with 70 grn Nosler ballistic tips.