Thread: Chamber/action cleaning

Ahhh... okay.
What make barrel?
I would try to colour a case with a vivid and try to chamber to see where it is causing the issue.
I suspect either the chamber is not to modern 300 BLK specs (I recall there were differences in earlier 300 specs), so the headspace (shoulder), case neck thickness or even the projectile is getting driven into the rifling early that is causing the problem.
Either way, it sounds like a trip to a gunsmith to check.