Re: .338 RUM problem
My advice is to get a larger target backer and shoot a few groups at 300 yards. Then decide if you have a problem. Sometimes the difference between 100 and 300 yards when it comes to lefts and rights, lighting etc....can suprise you. Try and eliminate any frustration or doubts from your mind when you do this test. If there are bullets sprayed all over the backer, then worry.
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.