If there are marks on the body of the cartridge case that are indents of some kind I would suspect the chamber has some kind of roughness in it, crud, machining marks etc. Carbon Maybe but not likely. I have a mauser that was rebarreled and the chamber was rough untill I smoothed it out (dont ask how, others on here would howl). Before I smoothed it out it was an effort to extract and I never load to max. as I find that is never the most accurate in my rifles. Whatever is causing indents that were not there on the cartridge before firing that may be your problem. Worth checking and cheap to repair. If you have a gunsmith nearby ask him to run a chamber reamer into the chamber just enough to clean it not modify it, assuming he has the appropriate reamer. If your marks are raised then you have machining gouges in the chamber and a reamer wont fix those. In that case a bunsmith would have to face off a few thouands off and re-ream the chamber.

El Lobo.