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One of my AR15 builds has some play between the upper and the lower. What's the best way to fix this issue (aside from buying a matching upper/lower in the first place)?
 
The quick and dirty way is this UTG Accurizing Wedge AR-15 - MPN: RB-A005

You could also emulate the aero lowers that have a nylon tipped set screw drilled and tapped into the grip lug to provide tension between the two halves. You would have to drill and tap the lower like the Aero and install this screw Nylon Tipped Tension Set Screw

I can post pics of an Aero lower with this screw when I get home later today and provide a measurement for location.
 
Sometimes they do require a little trimming, otherwise you'll have to pound your pins in and out.
 
What kind of slop? As long as the take down pins run tight and the build cycles and shoots fine, then a little bit of slop is OK. Are we discussing and cussing unsightly gaps between the upper and lower along with TIGHT pins or are we talking about wiggle between the two?

A solution is to buy matched upper and lowers. Matched meaning a very tight fit with tight take down pins along with zero or very very little visible space between the two then pinned together. A slight mismatch in color is OK as long as it is not greatly visible. Difficult to obtain.

The fix with sloppy fit is to try another combination of upper and lowers. Difficult or impossible with the home build. We matched 10 combinations at a time to get the best fit. The result was that we always had a couple of lowers and uppers that required timely hand fitting. Easy.

But time consuming. A rubber or plastic wedge thingie is not the way to go. You want proper tight fitting upper and lowers. A helpful hint for very tight take down pins is to grab and simply squeeze hard the upper and lower together then pushing out the rear pin to open the build.

Again, easy to learn and do. You squeeze the two together then push out the back pin to open. The best solution is to pay the money and buy two receivers that have been hand selected to work together properly. No slop. Very tight. With time they will loosen up a bit.

Close to a joke. Hundreds of AR15 builds plus uncounted many M16 quickie rebuilds but all long ago and far away. I am now just old and cooted.
 
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FYI, the Accu-Wedge will only fit "low shelf" receivers without major modification. A "high shelf" receiver would require trimming off the majority of the Accu-Wedge to fit under the takedown pin.

Here are some photos showing the Accu-Wedge, an Aero lower with tension screw, and a "high shelf" lower receiver.

IMG_20190205_164304.jpg IMG_20190205_164234.jpg IMG_20190205_164404.jpg

Hope this helps!
 
Strange I have not seen this sort of thing yet. I have Anderson (Poor Pony) lowers and I have fully assembled Troy upper receivers and they stay tight. I have fired them about 400 rounds with one and over 600 rounds with the other. I admit that the one black Troy upper I need to use a punch to push the back pivot pin out. Once the end is loose then I can pull the pin out with no problem. Putting it back together I use my fingers only.
 
JB Weld...

Fixes just about anything duct tape doesn't ;)

Sure, that and black shoe goo and you are set buddy...:D. I was having issues with my rear differential cover leaking out synthetic fluid on my 04 gmc. Used a new gasket and a small amount of shoe goo on it and it never leaked again...:p
 

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