Big Stick, I think that you missed something important. My M-700 338 RUM has a really sloppy chamber. I expand virgin cases to 35 cal. and then neck back down leaving a noticeable false shoulder. After shooting once, I add two grains of US-869 to regain my load density.
There are two ways to soup up the factory 338 RUM. First, use a custom barrel and a minimum chamber. Second, stretch your magazine and seat bullets out farther. As usual, Remington made some compromises, to be able to peddle these 700's at a profit.
But doing both of the above mods, means that you have commercial ammunition available, and the legal 338 R.U.M. legend stamped on your barrel for "wheels up" hunting trips. All of these brass cases will have 338 RUM head stamps to go along with the barrel legend.
The 338 RUM has earned a reputation for accuracy, as is. Remington has said that they will never sell these RUM's as Basic Brass, and trying to push back a 300 RUM's shoulders to make a longer necked 338 RUM, which might help, is very difficult to do, without crushing shoulders.
If you gotta have that last little bit of extra poop, then simply go with the 338 x 378 Weatherby. When I remount a 338 RUM barrel, it will be one of these custom barrel/chambered jobs. And the forward sling swivel mount will be soldered onto the now 25 inch long barrel. You live, and hopefully learn, along the way. IMHO.
