That's the same size as my 9.3x57 was, Matt- .370" groove diameter. The bore was a standard .358".

The neck in my chamber would only allow a .367" bullet seated in an 8x57 RP necked up case, though. It was fine for .365" to .367" jacketed bullets and very accurate with those, but would need to be enlarged by .005" to allow larger bullet to be used, to .397"- just a guess.