Everyone above gave you good advice. I would be willing to bet that there is one of two or both things going on here. (besides shooting off a solid rest and putting the barrel and not the forearm on the rest/bags) 1. Improper scope installation or faulty scope. and 2. Inconsistent shooting technique. (rifle position, cheek pressure, grip etc.) Consistency is the key to accuracy not matter what you are shooting.
And as I type this there could be another factor here that you may or may not admit. But it could be that your 300WSM is just too much gun for you and you're flinching slightly. But we will never know that unless we watched you shoot.