Hornady published load data for the 338 RCM in their most recent reloading manual. Since I have a Ruger in 338 RCM I was eager to check out the data and I find it somewhat interesting when compared with Hornady's own data for the 338/06.
Now I have used water to check case capacities between the two. The 338 RCM holds about 3 or 4 more grains than the 338/06 (A-Square version). However, looking at the load data in most instances Hornady shows heavier powder loads in the 338/06 than in the 338 RCM. Also, load data I have found on line show heavier loads for the 338 RCM than for the 338/06 with the same powder and bullet weights.
So what gives?
I have already safely exceeded the Hornady 338 RCM data with my handloads. Is Hornady keeping the loads low on purpose so that their factory loads look all that much better? To my mind the 338 RCM is nothing more than a shorter, fatter version of the 338/06 Improved. Both are good cartridges, but with the paucity of load data for the 338 RCM I'm not sure Hornady isn't doing it a disservice by essentially loading it down.