My experience with Sierra bullets is they like to jump. My 175g SMK 308 load jumps 40 thou.
I would first load the minimum powder load and load 3 at it 3 at 0.5 grain more, 3 at 0.5 grain more....till your at max load. See what groups best out of that. You may be hitting them hard enough that it has the barrel harmonics jacked up
Like said above check your runout, scope/sight bases, action screws torqued? . Do you have another rifle to shoot to see if it may be your technique ?
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