Welcome to GBO!! I'd double check to make sure the base and rings are tight, did you use loctite on the base? If that's all good, I'd try some different ammo, it may never like the Win ammo, but a simple change to Remington or Federal may be all it takes. Federal Power Shoks and Fusion have a real good reputation for being accurate in mine and a lot of other's rifles, even inexpensive(if there is such a thing any more!) Rem core lokts shoot good in a lot of my rifles.
I'd also put the front rest under the front of the frame instead of under the forend, that alone can cause accuracy issues in a break barrel due to the forend being supported at the mounting stud, and not floated.
Tim