Yesterday I went to zero the rifle, from a rest, no pressure upon the upper at all, not even from a sling. It grouped very well, but very far to the right.
The rear sight has 40 clicks of adjustment, 20 to the left, and 20 to the right. I have the rear sight aperture 12 clicks to the left to zero the windage. Elevation was no problem, almost spot on from the get go.
I do not think that this is due to a canted front sight base, or an improperly milled picatinny rail on the upper receiver.
I took a piece of thread and fed it through the small aperture of the rear sight, and looped it around the front sight post, and pulled it taught. I then took photographs, and oriented them using the photo software with a grid so that the milled slots from the picatinny rails meshed perfectly with the horizontal lines on the grid. The string made a perfect straight line that meshed with the vertical lines on the grid.
The string seemed to stay proportionally in the middle of the barrel on multiple pictures (to attempt to take into account paralax errors), indicating that the line of the sight through the sight picture does indeed go over the middle of the barrel.
I therefore believe that the front sight/gas block and upper receiver are indeed square to each other, but that the rear sight is not manufactured properly. What is your opinion on this matter?
Full size and quality images can be emailed upon request if you want to see the high res photos, but below is what photobucket will let me put up.