Originally Posted by
7 RING
Place a level across a flat on the upper receiver and see if the rear sight assembly appears canted to the right when the receiver is level. If it is canted, you have found the problem. Try a carry handle off another AR-15 and see if you still have to adjust the sight the same amount to the left to zero. If you do, the front sight is probably canted to the left and you will have to find someone with the tools to reindex the barrel.
I was told by a major AR manufacture that 13 clicks from mechanical zero on the rear sight is acceptable. I do not find it acceptable and make adjustments until I am within 2 clicks of mechanical zero on the rear sight. I like to have the extra windage adjustment available to make corrections when I am shooting at distance on a windy day.
Good luck and let us know how it turns out.