Not to take away from what you say regarding the 338 RCM being a "338-06 Improved" (it has approx 3.5 gr's more water capacity), but necking 338-06 brass from cheap and available 30-06 brass is the simplest thing in the reloading world... and if you want 338-06 A-Square brass, it's available.

I still am confident the 338 RCM will be in the trash heap of cartridges long-term... heck, even short term.

“Perfection is Achieved Not When There Is Nothing More to Add, But When There Is Nothing Left to Take Away” Antoine de Saint-Exupery