Did you pull the bolt, point the muzzle at a light and do a visual inspection?
What did you see??
I ran into the same thing with my son's AR-15 chambered in 6.5 Grendel. It wasn't shooting "right" last time at the range so I checked it out for him.
I was cleaning it for him and felt a restriction just ahead of the chamber when running a patch through the bore.
Pulled the bolt and there was this ugly looking black ring there- I thought something had torn up the rifling it was so thick.
Turns out it was a very hard to remove carbon ring- hardened...it took a lot of scrubbing/brushing with JB Bore Paste, and Carb-out, to get rid of most of it. I still see black discoloration, I think it's just the stainless steel discolored because the rifle is back to shooting sub-minute.
The location- just ahead of the chamber- suggests that to me... it'll be easy enough to see if that's the case.