Tyrone, would horizontal stringing be from poor bag technique?
Yes, it's a notorious problem with ARs because of the bottom angle of the A1 & 2 stocks and various collapsibles. It's the reason dedicated BR guns have a straight stock.
I'm not trying to be condescending or Capt. Obvious, you've obviously figured out how to shoot an AR off a bench. I just say this for the audience. BR is definitely a skill that has to be practiced and having the correct equipment helps.