I have never seen an over-gassed barrel not shoot because the gas-port was simply too large.
You could have one of many problems: unsquare upper receiver. Do you get point-of-impact shifts when switching bullet-weights? Did you have to adjust your scope a significant amount to get it on paper? If yes, then I would get the receiver squared.
Assembly: is the barrel and gas-block free-floating? If you remove the BCG and roll the receiver over, can you move the gas-tube a little in both directions, in other words, does the gas-tube float?
The other would be simply a bad barrel.
If you want, send me the complete receiver. Remove the optics, but send everything else. I will disassembled it, square the receiver, borescope the chamber and bore, and reassemble it the way I do my full custom builds. If this fixes the problem, then you will be lucky, and all I would ask is that you pay return shipping. If this doesn’t fix the problem, I would be happy to fix you up with a Barrel that will shoot.
