The only thing that makes me think that isn’t the problem, is that the rifle never did this the first several hundred rounds. It always shot the first shot the same place as the 5th shot, within an moa or so.I have had several rifles just like this and can pretty well assure you that the spine of the barrel is not located perpendicular to the rifle. Most of the time production rifles are close enough to being straight that companies do not bother with it but every one in so many are going to be warped more than acceptable. To correct this you will have to unbarrel your rifle and align the spine, this will require cutting the flange on the barrel and probably reaming it again. Happy shooting.