AR shoots 55-gr low and left, why?

[ironworkerwill]

varmintAl.com has some computer animation of barrel harmonics Yall may like. barrels do weird stuff when fired.

 

[JohnKSa]

I've seen plenty of left-right POI shifts between ammo types in my own testing at 200y.

Right, no argument there. The question was whether it was due to a difference in recoil characteristics or due to a change in barrel harmonics.

It would be pretty easy to try shooting a few groups off the opposite shoulder. That should have zero effect on barrel harmonics, and virtually no effect on elevation error due to changed recoil characteristics. If the windage stays the same, that would be strong evidence that it's not related to recoil-related issues. If it does change significantly then that would be pretty good evidence that it's related to recoil.