Have a Mesa in 6.5 Creedmoor that exhibited the same issue, would not group at all. Sent it back to Christiansen and they sent it back with target groups under 1 moa. Tried again and still would not group. I finally decided to check the stock to action fit. Found that the barreled action rocked in the stock even though they had skim bedded. I shimmed with electrical tape at different locations until I found where it was rocking. Turned out the recoil lug recess was to shallow causing the recoil lug to bottom out and as the lugs were tightened it was putting action in a bind. Removed material until the rocking stopped and then bedded the action back to the tang to 1 1/2" in front of recoil lug. Result was sub 1/2" moa groups. Not saying that is what is wrong with your Mesa but can tell you what I went through. Good luck.