Is it possible it is a 9.5X57 and the barrel is marked wrong?
That sounds like a more logical explanation. It's OK if it is. The 9.5 is a fine shell too
obturation of the .366" up to 375" seems like an awful lot to "bump-up" but it may be possible due to the weight of the bullet. Such heavy bullet have a lot of standing momentum compared to something of ...say...30 cal. Maybe they are obturating enough to fill the bore. I'd slug the barrel and measure to be sure it is not a .375" instead of a .366"

I make a 9.3X57 for myself on a 98, and my shells come out with only a small amount of neck expansion.