Any chance you took off a 0 moa rail and replaced it with a 20 moa rail? I have both and almost made that mistake just last week putting together a 6mmBR.

Another possibility...how many foot pounds did you torque the new barrel to? Did you use a new nut and recoil lug such as the precision ground ones, or was it the originals?

I removed the barrel from my 22-250 for a re-crown, and found the action face was worn in an irregular pattern. Dropped that off at the GS for truing and ordered new recoil lug and nut from NSS. BTW, both the 250 and 6mm are X-caliber barrels from Rosco.