For those interested, and take this for what you paid for it, I made up a 375 Dakota case and what I thought Jeff might do with the 375 AR (2.1343-2.2450" shoulder, 2.65" long, 3.35"COL) on RCBS.load's cartridge design software, and then seated a 1.2" bullet in each to 3.35"COL. The water capacity underneath the bullet, best I could tell was 95.7g and 99.5g, respectively.
I don't know how accurate this thing is, but I've seen it come pretty close, so I'm thinking this case is good for about a 3.97% increase in useable powder room under a 1.2" bullet, more or less 270g. I could be totally wrong, but I hope it helps your question, vapodog.
Now, if you seat them both to the neck, the AR round goes out to 3.45" and gains almost 3 more grains against the Dakota's .5g at 3.37", bringing it to about a 6.55% increase.
Again, take it for what you paid for it....
Cheers.
Taylor