Zero with irons issue

[1986s4]

So I had an upper built using parts I had. A BCM upper receiver with a Colt carbine length skinny barrel [16"]. It runs great but I had to run the DD A1a rear sight all the way to the left to get it on target [36 yard zero] . I don't doubt my gunsmith who put it together or the parts, all quality. So my question is: what can be done to get a zero more centered in the sight body? I have a Magpul plastic rear and a Colt A2 carry handle rear to try out when I get the chance.

[ndmiller]

Sound like something is off-center, over/under torqued or bent. Is the upper with FSB or flip up front sight? I'm sure there is a sound procedure to follow in this case, someone will chime in with it shortly.

[Circle_10]

It’s possible that maybe something is janky with the way your rear sight is sitting on the rail or something so it’s worth trying at least one other rear sight - either the Magpul or the carry handle - just to see what happens. If you get similar results your FSB might be canted, or the barrel indexing notch on the upper might not be positioned properly. Do the feed ramps in your barrel extension appear to line up properly with the ramps in the upper?
I’ve also heard that during production, it is possible for barrels to sometimes end up with a bore that is off center. That seems like something that would normally be caught by QC, but sometimes lemons slip out.

[1986s4]

The barrel has the standard Colt FSB and was previously mounted on a carry handle upper that did zero properly. So, as far as I know the barrel is within spec. I will check the ramps to see if they line up as they should. The BCM upper was bought new by me and used very lightly in a previous build, it also zeroed as it should. I am thinking:
1) Try a different rear sight
2) Take it to another smith for a 2nd opinion/adjustment.

And thank you all for taking the time to help.

[jsbhike]

Long shot(especially since the parts zeroed previously) to look at some point after a new sight, but the receiver face and/or barrel extension shoulder may not be true and you are getting a stack of dimensions cocking the barrel left.

[T2C]

You can use the A2 carry handle or reverse the DD sight and see if the same problem exists. If you still have to adjust the rear sight all the way to the left to zero, the FSB may be canted slightly to the left. The barrel may need to be re-indexed, which is relatively easy for a gunsmith or armorer to do.

[bamashooter]

If all is kosher you may need to face the front of the upper. Easy enough to do. Windage all the way to either side is within spec if zero can be achieved. Stupid deal to me. To me they might as well take it all the way and state "If you can hit something, it's all good".

[ViniVidivici]

I'd say the rail on top of the upper is out of spec, the rear sight is, the face of the upper isn't perfectly perpendicular/true, or the notch for the pin is off.

Normally I'd say canted FSB as well, but I don't think so, as it was GTG in another upper.