That wiggle between the upper and lower is something all AR owners have encountered. In fact, some amount of slop is to be expected. None the less, it still bugs me.
There are various doohickies for eliminating the wobble; Over sized takedown pins, the O-Ring hack, the wedge, and shims. Personally, I like shimming with painters tape.
Then there are receiver sets that are matched, I have one such set, milled from billet aluminum, and even that has some slop. And then, there is the Frankenrifle, my last build made of spare parts, those receivers lock up tight like they were made for each other.
Some say slop is necessary for reliable operation, others say the wiggle diminishes accuracy. I can attest that slop reduces accuracy, but will the increase in accuracy matter to most shooters?
My SPR was shooting quarter sized groups at 100 when I built it, then I shimmed the receivers and tightened up the groups to dime sized.
So, what is your take on slop, does it bother you or is t a non-issue?
For me the accuracy I expect from a rifle depends on the purpose. For most applications for an AR, I don’t expect to need to hit a guy in his left nostril at 200 yards.
Military combat rifles are expected to be 4moa or better. I’m happy with up to 2 moa as that is about the best I can shoot off an improvised rest.
If I want to shoot tiny groups far away, I’ll reach for my bolt gun in 6.5CM using a bipod and squeeze bag.
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A simple rubber O ring has cured that problem for me on several occasions!
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Were you using the same ammo//load on both occasions?
I suppose if the slop is so bad maybe you aren’t able to hold a tight group if you’re wobbling around too much. In my Army days we qualified with the most beat up rifles and crappy steel cased ammo. Nothing was tight on these things and we still were hitting at 300 with irons.
As long as the upper is tight (barrel and sights properly torqued) then it’s a matter of the load and twist on accuracy.
I’m at the range weekly and I rotate through my gear as not to get bored with anything. I do find if I have not shot something for a while my groups may not be as tight as I expect or how I remember when I last shot it. But if I bring it the following week, magically things tighten up for me. It’s worse if I had been shooting rimfires for a couple weeks and then back to the AR.
I guess what I’m trying to say is that it’s the shooter not the gun 
Improving the fitment between the upper and lower may improve the shooter’s accuracy making things easier/more consistent but the rifle was already capable of that (with the proper load.)
It doesn’t effect accuracy all that much, but it’s an annoyance for me.
Most all my lowers have a set screw in them to take out the slop between upper and lower.
Yes, PPU .223 75gr OTM, several lots. Besides my reloads, that is all I shoot in the Mk12 SPR.
It wasn’t only two occasions, just before and after.
I haven’t noticed an accuracy issue but the rattle annoyed me so I use the o-ring. It breaks every few cleanings but they’re pennies each
Can someone link the o-ring to order? Some of my rifles are tight but the ones that are loose annoy the shit out of me.
Anything that fits around the upper lug will do.
When I get home I’ll get you some dimensions. May send you a few.
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Looks like I have #35 o-rings in my maintenance box but anything around that 9/16” ID should be ok as long as it’s not stupid thiccc
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I ordered a variety set of o-rings from Amazon. I think they sent the wrong set but I got ones that still work. The difference is amazing!
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