The issue that I have been running into, is that occasionally the rounds don't want to chamber all the way. The rounds that don't want to chamber properly usually come back out of the rifle with a shiny spot on top of the case, and the shoulder has a dark ring around it.
I reread your first post and realized I had missed that you said the shoulder had a dark ring around it.
If you couldn't chamber the round and fire it then why the dark ring? The only thing that comes to mind is carbon in the chamber and/or your shoulder is not bumped back enough, so the case rubs some of that carbon off, or the carbon has built-up in the chamber's shoulder enough to prevent the case and bolt from going into battery.
After making sure the chamber is clean does it still not go into battery? Also you mentioned a shiny spot on the case. And dmazur asked you where the shiny spot was exactly.....you didn't enlighten us...so where was the shiny spot?