Almost any compatent smith who can PROPERLY ream a chamber should be able to punch out an existing 7 Rem to Mashburn. Some may want to turn off some part of a thread to get a good bottom on the belt, otherwise it will clean up the chamber nicely. The other thing is to make up a dummy round for proper throating. The original Mashburn spec. had a short COL. Mine is set up for 175's seated with the base of the boattail at the base of the neck. That gives a COL at 3.6" with a Sierra 175 GK. One of the nice things is that with a long neck you can load all your bullets down to 140's using the same COL and keep the same jump to the lands.
If you decide to throat for Bergers then use a smith that knows how to throat for Bergers or Matchkings and set the gun up to run those bullets with your max. box length, +/- 3.65"
Given the profile differences between Bergers and Sierra Matchkings, the throat should be pretty differant from normal ogive bullets such as Hornady Interlock, Nosler Pt & AB, Sierra GK, etc.
