Could there be a difference in the design of the MS rifling that upsets .263" bullets into the larger groove diameter to give obturation?

I've long thought there was something strange and deep-looking about the 1903's rifling, beyond the fast pitch. And no wonder - with a bore of .256, a groove of .268 would be about 50% deeper than used in most American rifles/calibres. Maybe something has to give.

BTW, the 30-year-old 6.5x54 RWS factory loads I have here show a diameter of only .2615" just ahead of the cannelure and case mouth. Though other calibres miked at that point sometimes showed half a thou reduction, these 159-grainers are pretty straight for a fair way. My guess is they do not have a .267" body dia.