I have a NIB Colt.  I installed a Samson MRFS-M.  On this rail I mounted a Troy flip up front sight.  On the reciver upper went a Troy BUIS.  In order to obtain a zero I have to crank the windage on the rear all the way to the left.  I have tired the following: reseated barrel ensuring proper torque, new barrel nut,  Troy rail system, new Samson rail.  All result in the same excessive windage requirment.  Regardless of the rail system used the upper rail sits about 1 mm to the left of the upper reciver rail.  

I think the face of the receiver may be uneven.  By that I mean where the barrel  meets the upper. Can this be fixed?  What are your thoughts?

Jim

Can you take some detailed pictures and post them.  It might save you a lot of confusion in the thread.  

+1 on the rail being slightly off center

Quoted: While the rail might not be installed correctly, it is also 100% possible that he receiver is not square and true. C4

Sure it's possible, but I'd put my money on the rail being on incorrectly.

If the receiver is crooked, you should be able to clampo a straight edge (like the edge of an 18" or 24" level) along each side of the receiver's top rail and measure a different dimension between it and:  the barrel, the MRF rails.  

Gentlemen,
I do appreciate the replies.  I have attempted to install the rail several times, each time using various means of stabilization i.e. a removable carry handle, ARMS #35, and a LaRue Aimpoint mount to bridge the gap and keep the rail true.  All have failed.  Once I start to tighten the screws they pull the top rail to the left.  This is evident even with modest torque.  Keep in mind I have tried three different rail systems.

Mike, I like your idea and will try it.  However, I believe the upper to not be square at the very front where the barrel extension touches the barrel.  I have seen This Tool

Have you ever used one to good effect? Am I way off here and simply paused at the precipice of destroying an upper?  I am willing to spend a few $$ for the tool.  If it doesn’t solve this dilemma I will not be out much but  don’t want to destroy an upper.  Your thoughts?

Jim