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While I do not think that this is necessary for a properly machined lower, I have had folks ask me about them so I figured I'd post this.

I set the mill up like this, using the following datum points. The normal "Y" axis zero, the flat area just behind the PG mounting boss for the "Z" zero and .156 (half of 5/16") to the right of it as the "X" zero.





Using a 5/16" end mill I machined along the X axis .265 to the left into PG mounting boss. You do not need to mess with either the Y or Z just leave them locked.







Drilled it out with a #3 bit.





Tapped it to 1/4"-28





Added 1/4"-28 X 3/4" set screw. It sits well below flush when fully installed

 

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15,204 Posts
So the upper fits tight and dosent flop around killing accuracy and wearing pin holes out

My only concern is how fast will that screw wear into the upper?
 

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NoWorkCamp4Me
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5,546 Posts
Takes the wiggle out of a miss matched upper and lower. In my opinion a feel good thing. I can't see it helping score higher on paper.
I prefer a tight fit with no air gap with upper and lowers. The gap in my mismatched Anderson lower Sota Arms upper I can slide a strip of 20 weight printer paper thru.

Sorry Biggun. I type slow.
 

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I doubt any loose part helps accuracy.
Hammer swing and hitting the firing pin alone can move a loose upper.
Various harmonic changes from Wichita way the upper is resting can't help.

I have never seen a super accurate AR with a loose upper.

Many guys only need minute of man so it won't matter.

I note many down play this issue . It's often the same guys who will insist on a free float tube and a high end optic and then shoot Wolf ammo.
 

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gunco irregular
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I remember seeing an article on the Military shooting teams and mods to their rifles. They did the same thing basically by bedding the rear takedown lug while keeping the action open a couple thousands with a feeler gauge.
 

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The military teams do quiet a bit to there gun.
Different twist rates,chambers,and custom barrels.

Part of the whole M16 is so accurate myth.

A lot can be learned from there work.
 

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in the blueridge mtns
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Thanks for the repeat! I've only done two and used a set screw with a brass tip on it. Good solutions should be repeated time to time. Helps the hobby.
 

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NoWorkCamp4Me
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I cut a piece of 1/8th thick gum rubber from a sheet I was using to line the bottom of one of my roller tool box drawers. I glued the small piece to the bottom of the rear upper takedown pin post on the rear of the SOTA upper receiver. No more wobble on my SOTA/Anderson combo.
 
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