I am doubting it has to do with the barrell nut simply because it is twisted to the right (looking down the barrel). I did not notice it until I mounted my eotech and could see it was canted in the window. Then when I went out to the range to zero the irons, the rear aperature had to be pushed almost all the way to the right. I am still considering removing the FSB and installing a flip up front sight which will give me the ability to adjust it. Fron inside the upper reciever, the barrell extension apprears to be sitting perfectly. Other than the sights being canted, it feeds ammo very well and once the eotech was sighted in, it was quite accurate and consistant. I found that with the GG&G BUIS at the rear, the front sight post is a bit too short, so I'll need a longer one.