Load 6 and shoot the first round into the berm and a 5 round group. If you still have a flyer it's the shooter, if not it's probably a placebo effect and was always the shooter.

Built my father (80+) an AR and he was convinced of his first round flyers. I asked him during one range outing if I could fire the first round, he said yes, but wasn't paying attention that I never fired a shot, just loading seating the mag and then getting up after others fired their guns. No flyer on that group from him. When I told him of my deception, he declared I had loaded the 1st round (although he did but forgot) and changed the gun. Until he started adding junk for more and more accuracy (why I'll never know) he was consistently 1 MOA @ 1-300 yards benched on bags with irons (fundamentals). After adding scope 1-4 Leupold, bipod, rear bag, cheek rest, flashlight and mount (why?), brass catcher he never shot better than 2-3 MOA as it all got in the way and hindered the fundamentals.