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Old January 2, 2020, 05:26 PM   #8
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Join Date: November 13, 2006
Posts: 6,617
I'll agree it can be about the upper receiver face,but thinking about the machining operations I'd guess (I don't know for sure) the barrel extension counterbore,barrel nut threads,and receiver face are all cut in the same setup. Likely the BCG bore,too.

Anything could happen,but I'd like to think the receiver face is better than that.

IMO,lapping is about getting it flat,knocking off a high spotso the barrel extension has a stable seat.

I'd be more suspect of indexing. How precisely is the front sight base/gas block indexed to the barrel extension pin that indexes the barrel extension to the upper?

While maybe its not supposed to happen,early on I used the little blue GI armorers booklet to build. I used the GI type armorers vise jaws to clamp on the barrel OD to hold the upper as I torqued the barrel nut.

Which means I was holding the barrel,not the barrel extension.

Murphy's law being what it is,I had a barrel,extension turn.

In fairness,the military process of setting a barrel extension is not the same as setting up a match barrel.

Don't do that with match grade barrels. So I bought the long bar that fits through the upper and engages the locking lug recesses. No more problem.

I think there is a reason why the set screw type NM gas/block/sight base can be adjusted. So you can tune that.

Ideally,the gas port is drilled to be in a groove,rather than cutting a land.

So the setup for pinning the gas block has to consider indexing to the rifling twist.

I don't think they figure being OCD about visually centering the sights into the battle rifle tolerance stack up on all that.

Last edited by HiBC; January 2, 2020 at 05:31 PM.
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