Have you Really cleaned the barrel??? If you have shot it more then 20 times between cleanings on a new barrel you are way behind the copper build-up.
Have you looked at the muzzle of the barrel for any small nicks or bad edges on the rifling?? A small nick or ding on the rifling will throw your bullets all over the place.

Just some thoughts

I have had some of my best .243 groups with 100gr Sierra SBT's and a full load of IMR 4350 up to 43.0 grs

22 Mag