I thought the K31 has a .308 bore vs. .311 for MN 7.62x54.
Technically the K31 is a .308 bore. The M/N's can be anywhere from .308 to .311-.312. If you have a M/N you need to slug the bore to find out for sure what your bore size is.
My M38 prefers the .308 bullets where one of my 91/30's prefer the .311 bullets and won't shoot the .308 bullets worth a crap. My M38 won't shoot the .311 bullets worth a crap.
Even a lot of the .308 bullets don't mike out to .308. The Czech silvertip mike out to .310 so there isn't any pressure problems that I have seen.
My K31 doesn't like the .308 bullets(168 gr. Barnes Triple Shock) my M38 likes and my M38 doesn't like the .308 bullets(165 gr. Hornady Interbond) my K31 likes.