I just got done converting two Mark-X actions, one a 458 Lott and the other a 416 Rem Mag. Both started life as magnums, a 458 Win and 375 H&H. The 375 was converted to a 416. One thing to consider, the standard bottom metal for the Mark-X will not hold more than three down and that is iffy. I used Weibe bottom metal which will hold four down and one up. No small task. Not only the rails but the underside of the action had to be machined to utilize the ovesize, wider new box. It feeds flawlesly with both pointed and flat nose solids but with a lot of work on the rails. Not cheap either.
