The original German military designation was 8mm. 8x57j was the civilian designation. 8mm is .315", and that is what the original groove diameter was. The bore diameter was, and remained, .311". When the shallow grooves wore and eroded too quickly, the military changed the groove diameter spec to .323" diameter, deepening the grooves to .006" (.323"-.311"= .012"; divided by 2 = .006")
Then, for some reason, even though the 8mm had been made 8.21mm, the military chose to call it 7.9mm. This is often seen in American publications as 7.92, but the official German documents I have seen always call it 7.9. The civilian designation is 8x57js.
Jim