I have a mauser rechambered to 8x60, I fireform 8x57 brass to use in it. Short neck, but enough to hold the bullet squarely and tightly.
Check your fired brass against an unfired round and look for any difference in shoulder height or short necks.
The bore being .318, "might make a difference and might cause this problem". Personaly, I doubt it. Especially if you're useing commercially loaded ammunition.
I've seen several mausers with .318 bores, that have had literally thousands of rounds fired through them. Most of it WWII milsurp stuff, that was loaded far hotter than any commercial North American stuff.
It didn't cause any lugway setback or even any headspce problems. We're only talking .005 in. Not enough to really worry about, unless the rifle is of questionable quality or strength in the first place.
I once associated with a fellow that shot surplus 8mm fmj through a pattern 17 that he had picked up for $5 at marshal wells along with a huge quantity of 8x57. He and the store clerk assumed that because it fit in the chamber, it must be correct for the rifle. He did complain about the heavy recoil and poor accuracy beyond 50 yds, 6in at 50 yds was acceptable to him.
The P17 didn't suffer any ill effects from the 8mm onslaught and beating, no headspace issues, hardly any fouling issues and because he knew about corrosive primers was very careful to clean the rifle properly. His biggest complaint was the 8mm cartridges didn't have a rim to fit in his No I MkIII enfield. Very penny concious fellow by the way.
I gave him a box of M2 fmj to use in the P17, but he saved them for long range use (100yds) and what the he--, the 8mm worked just fine for his needs and there wasn't any reason to waste it. He ran out of the last of the 8mm 2 years ago, after shooting hundreds of deer with the P17 and guess what, the headspace in the rifle is still bang on and the bore is still shiney and bright. To bad he hacked the bbl off with a hacksaw and filed the muzzle flat and square. It still is acceptabley accurate out to 50 yds, even with the filed screw that is the front sight.
I tried to get a pic of the rifle but for various reasons, I was declined the opportunity.
bearhunter