its true for the 7,62x54R also which must have a 7,62mmm bore to a 7,87mm groove diameter. when we remember that the 9x57 have a bullet diameter of 9,08mm and the 9,3x72R have a maximum bullet diameter of 9,57mm there is at least some kind of a system in the russian style.

I am not a fan of plastik stocks and weaver mounts Sarg but the rifle you have post looks very good and it did the job as you say. must have been a dragunov muzzle break and together with the very pc bended bolt it have some elegance.
got work and miles away from the bubba victims.