Had a similar issue with a recent 10.5" LMT build I did; when sighted in, the rear BUIS apeture was biased significantly to the right and being an anal SOB, I didn't like it. Figured since the rear was off to the right, the barrel was biased to the left when the nut was tightened down. So, took everything apart and placed a really thin shim between the barrel and receiver so when tightened and torqued, the barrel would not sit in the receiver, 'pointing' towards the left as much. Went back to the range and sighted in, and voila, the rear BUIS apeture is perfectly centered - guess I guesstimated the correct amount of 'shim' to nudge the barrel. BTW, I did check the FSB and it wasn't canted, so figured it was the way the barrel was 'sitting' in the receiver.
I too don't think it's the BUIS - it was nice of MI to send out another BUIS to try but in all likely hood, you still may have that rear apeture off to the side. If you're still not on target with the apeture pushed to the limit, you'll either have to shim the barrel with something 'substantial' or send the receiver back.
Dave.


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I personally think that it may have had a twisted barrel.

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