Heres a link to an article on 6mmbr that shows all the 6br variances(including the BRX) and their differences. At the bottom of the page it tells how you can headspace/fireform those type of improved chambers. Looks like a BRX has a shoulder pushed out .010", so forming a false shoulder seems to be the way to go, according tho this link.

hope this helps.

http://www.6mmbr.com/6BRImproved01.html