I'd clamp it and see if the crosshairs move when you parallax and/or change mag, and when you dial for 10yds...
I assume you're shooting with a bubble to rule out can't as well?
I'd also suggest doing it indoors will rule out wind. You need a lot to move it 15mm at 10yds, but it's easy to think it's right further out and then when you come back it's wrong. But if it's only doing it at 10 and every other range is smack on i'd suspect the scope. If it gradually wanders i'd suspect windage.
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