I'm no expert either but I have owned and measured, (slugged), several Mauser sporting rifles, pre-WWI and between the wars. All I have measured we definitely knew were pre-WWI were .318 groove diameter. Those from between the wars could be either .318 or .323. It seems those we were positive were proofed closer to the end of WWI were more likely to be .318 and the closer they got to the beginning of WWII for a proof date, .323 seemed dominant. That's just the experience of one guy and certainly not definitive.