I know you said a clean barrel wouldn't group but have you tried a good bore paste on it? I only say that because my Model 70 30-06 got to where it wouldn't group the old IMR 4350 / Hornady 165 gr SPBT combination and I kept it's barrel very clean, I thought. After trying several things, like you, I did the bore paste and it immediately went back to the tight groups I was used to. Copper build up can sneak up on you. If it's a new rifle, I saw where they are making them again, it can be full of factory gunk so the bore paste would still help.