Help with Accuracy? AR-15 with 20 inch thin barrel.

I built this rifle, its my take on the WWSD project, the major differences being a receiver designed void of a forward assist in the first place, narrower CODA handgaurd, and a 20 inch barrel instead of a 16. I believe this is more in line with Stoner's vision, especially the barrel being this length. (ignore the california maglock on it, its cringe but fin grips make my tism hurt.)

The problem is that the thing shoots like 5 or 6 MOA. I am not the best shooter around, but 100 yards with the 5x prismatic I have on it is really a piece of cake, I printed better with an M1 Garand with open sights the same afternoon. The optic is tight, the barrel nut (which is proprietary to the CODA handgaurd) is tight and to spec. The gun is a little bit overgassed, but that does not seem relevant to slow fire accuracy at a relatively short 100 yards.

Relevant Parts:

-Faxon Gunner Profile Barrel, 20 inch. 1:8 twist, 5.56mm NATO.

-CODA Evolution Lightning Carbon fiber handgaurd with barrel nut, (13.5in)

-Aero Precision Upper Reciever, No forward assist.

-Young Manufacturing BCG, all chrome, with HMB Bolt (basically has a rounded off cam pin and the cam pin hole doesn't pass through the whole bolt.)

-CMMG gas tube, rifle length

-Pretty sure the gas port is also CMMG, .625 (triple set screwed)

-Surefire 3 Prong open ended war comp.

Something of note, the dispersion seems mostly horizontal.

My main two theories are either a problem with headspace, or barrel whip.

Maybe its barrel whip, after all many short rifles are more accurate than longer ones purely because the barrel flexes less. However, while the 20 inch gunner profile barrel is a thin barrel, but not a pencil one. (it does narrow to that after the gas block) furthermore, plenty of full pencil barrel rifles are 2 or even 1 MOA capable no problem.

Headspace may be an issue, but this BCG is a good one and is the same choice for the WWSD guns as well, which also pair it with a Faxon barrel. (16 inch pencil)

EDIT: forgot to mention ammo, I shot it with both 5.56mm X-tac XP193 made by PMC (55 gr), and 62 Grain Green tips by the same manufacturer. The results were the same. Unfortunately did not have heavier loads that day, but its 1:8 it should work.

I am pretty new to building ARs, and this is a real head scratcher. Any ideas?