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Discussion Starter · #1 · (Edited)

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Just curious, but what is there to lap in the upper receiver? The BCG is a loose fit in the receiver.
 

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Just curious, but what is there to lap in the upper receiver? The BCG is a loose fit in the receiver.
It looks like it is for lapping the surface where the barrel meets the receiver. I guess that could effect accuracy if the surface were not true and placed uneven stress on the barrel.
 
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Discussion Starter · #4 ·
It looks like it is for lapping the surface where the barrel meets the receiver. I guess that could effect accuracy if the surface were not true and placed uneven stress on the barrel.
You nailed it. I stumbled upon this concept about a month back and I can't get it out of my head. I have a barrel and hand guard that don't like to line up together. I've get them really close, but they could be better.


https://www.ar15.com/forums/ar-15/-/4-506101/?page=1

BigBore makes some good remarks here.
 

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I have an Aero Precision Quantum handguard that I got really cheap, but due to the barrel nut design it was a real bear to get properly aligned. I ended up filing the receiver barrel threaded end down several thousandths and using one of the AP shim washers. Never thought about lapping, but made a mental note not to get another AP Quantum handguard. Their handguards require the use of their BAR barrel nuts which is the problem. Rather than numerous holes for the gas tube, if I remember correctly, there were only 8 places for the tube to go through. With the barrel nut all the way down, I could add every shim washer in the handguard kit and still not come close to aligning at the next hole, terrible design. I will not buy another.
 
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