If there is any shifting or movement in the upper, between the upper and the barrel or between the upper and the optics, accuracy will go to heck in a handbasket very quickly. Coming from bolt guns where any movement is taboo it is sometimes hard to get your head around the fact that movement between the upper and lower or between the lower and the stock does not have an affect. Those accu-wedges, been there, done that, are good for removing movement, but they actually push the two halves apart so while there is no movement you can feel, they do absolutely nothing for accuracy, the same as the JP tensioning device does nothing for accuracy even though it pulls the two halves together rock solid.
It is just one of those things you have to take on faith from people who have used them and have done their own tests with them. I sure have and did not believe the movement would not hurt anything either. Then after about $90 spent on several JP tensioning devices, and some more money on Accu-wedges, and shooting many groups before and after, there is no difference. With them, without them, Accu-wedge that pushes the parts apart under pressure to remove wiggle or the JP tension device that pulls everything together so there is no movement, the groups are not affected. And I did not try them at 100 yards, but at 200, 300, and 500 yards on several of my rifles. The movement just doesn't matter. I did not believe it either and bought all the sales pitches about the JP and the Accu-wedge, and that is all it turned out to be, sales pitches.
My match AR holds three inch groups at 500 yards, with our without either of the devices using the same Black Hills Molly 77 gr. SMK bullets. This is not from 40 years ago, but from only a few years ago when I bought my first AR match rifle and thought I could improve by removing the slop. Expensive lesson learned. Don't believe it, I welcome anyone to try it for themselves, but don't test with one group. The first time I tested it on my son's Colt HBAR, which was a loose as a goose, I thought I saw an improvement, for a group or two. By the time I had shot 20 or so groups, it turned out that first group or two were flukes, the average groups sizes did not decrease one bit. That was why I bought three of the tensioning devices. I shot a couple of groups on the Colt, thought it worked, and bought two more for the match rifle and another rifle. When I saw absolutely no improvement in those rifles I went back and retested the Colt, shooting 20 groups of 5 shots, then the truth was revealed. Those first couple of groups were a fluke and there was no change. Lesson learned, not everything you read on the interweb is BS.