Could easily be bench technique, setup.
I have been known to have excessive operator headspace occasionally
ARs aren't the easiest guns to shoot off a benchrest. The sloping stock makes them want to porpoise which string the shots vertically. A dedicated bench gun has a straight bottomed buttstock so the gun recoils straight back.
Makes sense. What do folks do it this case with respect to benchrest technique?
Front rest as close to mag well as possible. Aim and fire. Nothing fancy. Works fine to find out potential.
I do have a special upper to put match barrels on that is wide and slick and floated, to test 600 yard plus ammo and barrel before putting on a service rifle. There I take a bit more time on the rest part but nothing fancy.
