Are you saying 5 shots string horizontal 1.5"?
Does each successive shot move incrementally in the horizontal direction?
Such as: 1 2 3 4 5 or 5 4 3 2 1? If so, then barrel heating in conjunction w/bedding may be the contributor.
Or is it random, as in: 1 3 2 5 4 or some such order. If so it may be trigger pull, front/rear bag problems, inconsistent cheek weld or possible the bench if used is not solid.
If shooting prone you may wish to try to keep your body/spine parallel with the line of bore. That helped me a bunch with a large caliber cartridge even with a brake.
Just some suggestions..........
I may be the slowest guy on the mountain . . . . but . . . . I'm on the mountain!