My ARs in 6.8spc and 223/556 would shoot ok but never could get either rifle to shoot a group that I liked. The best I could do was around 3" at 100yds. Today I went meet my buddy at his range with them and had him shoot them because he is just an amazing shot. He shot the 6.8 first and sure nuff, the 3 shot group was about 3". He got to checking things out and he notice there was some play between the upper and lower. He took some kind of shiny tape and made a shim between the upper and lower and took all the play out. The next 3 shots was right at an inch. Got it zeroed and did the same with the 223 and it was throwing 3/4 in groups. We started playing and was singing steel plates out to 600yds. I'm really glad I decided to go meet him because I was really dissapointed in the ARs. Now I'm real confident in them. Amazing what just a couple little strips of tape will do.

yep. lots of little tricks. glad your problem was solved ...

Glad it got worked out, but little slop between the upper and lower shouldn't have affected the accuracy.

I thought the same thing but it dam sure tightened the group up and the shims was all we did different. Whatever we did to fix the problem, I'm glad we did. I was smashing clay targets at 200 and 300 yds every shot. I'm real happy with the guns now.

https://www.midwayusa.com/product/100340833/utg-accurizing-wedge-ar-15

OH yes it will, absolutely.
I spent hundreds of hours on the range trying to get M16 dialed in, some of them been shot a million times, with lots of slop.
You get that play out between the upper and lower it makes a world of difference.

I figured out that at least for me that i shoot fine with a loose gun and a light trigger. And bad with a loose gun with mil-spec trigger . I am thinking that right at the trigger break on the higher weight trigger i am ‘’taking up’’ the slack in the upper/lower and changing point of aim. I installed a wedge with the mil-spec trigger and shot well. Then installed a 3lb single stage cmc in the same lower without the wedge and shot well. Went back and forth 2 different times with the same results. My buddy says its all in my head. I told him i didnt care because it works. In other words ‘’I’’ shoot better with a tight gun.

I just ordered three rubber accu-wedges on ebay for $4.75. I noticed they also come in plastic. Too cheap not to at least give it a try.

Using a rear mounted (post) sight or scope?

Seems like I hold onto the lower, and if the upper moves when I shoot, since the barrel is attached to the upper solidly, it wouldn’t matter where a sight is mounted. It’s going to be erratic.

The lower is your anchor point. The type of sight or scope doesn't really matter. The tighter (and less whip action) you can maintain, between the barrel and anchor point, the greater accuracy potential.

I learn something new everyday. Never would I have thought this would effect accuracy, but I will have to try it out.

When you have to get guys qualified on the range, with old ass M16s, that should have been retired after Vietnam, you learn tricks.
I bet Marshmud's buddy learned some of the same ways I did.

You can also glue a rubber eraser off a pencil on the bottom of the rear take down pin lug. Trim it to fit where the lower will close and the pin slide in with moderate pressure.

Another trick is to use shim in .001" thickness under the receiver extension until then extension will just barely slide into the upper, Then glue it all in place with green loc-tite.

There are other tricks as well, but like the shim trick, they aren't as easy to do as wedging the upper

My buddy told me he usually uses the soft part of stick on Velcro and puts it between the upper and lower. He also told me that a little strip of gorilla tape on each side is usually enough to take the movement out.

I've used duct tape, chewing gum, even tooth picks.