I have heard that the early 7.92 x57 was in fact .318 bore . If this is an early "small ring" or 93 mauser or "clone" with a tight chamber the modern JS 8x57 that is .323 may not chamber as the neck would be at or beyond max.tolarance for a .318 bore .

Side by side with a 30 cal (of 308dia) the bore ought to be obviously larger or smaller .
If your mouser is a correct take from the gun you could have some odd 1 off wildcat or a "siamese" mauser with an
odd bore (7.5 or 7.7) which will not chamber 8x57 but would 7x57 .

It could also be that you have a bbl intended for very heavy or light bullets. I have a 1-12 308 that won't shoot 180's
and an 1-8.5 30-06 that won't shoot 130's . The 06' was a special order freak even in 1965 .