I know I'm wasting my breath, but...
When you shoot a rifle, the barrel flexes. If you have a stable shooting platform, the barrel flexes the same way each time. If you do NOT have a stable shooting platform, it flexes in random ways. While the barrel is flexing, the bullet happens to be travelling down it. The amount of rattle between the lower and the upper, as well as your bench, or body anchor, affects your shooting platform.
Now, we are not talking about "not hitting the barn from the inside" kind of inaccuracy, but it is one of those variables that accuracy nuts try to eliminate in their shooting. If we are discussing "minute of man" at 300 yds or less, don't worry about your rattle. If you are curious in inexpensive ways to improve your rifle's accuracy, there are a few things you can do that won't cost you big bucks to improve your rifle.
1. Eliminate play between upper and lower, as well as buttstock and pistol grip. All this can be done with a strip of duct tape or electrical tape. Remember that the tighter you get it, the harder it will be to take it down.
2. Free float the barrel (I think the AR is the easiest and cheapest rifle to free-float, with a free-floated forearm to be had for under $50 and can be just screwed on)
3. Post-ban barrels with unthreaded muzzles shoot better. The threading can actually cause the muzzle to bell out by a small amount, which won't be noticed by anyone except for accuracy nuts.
4. The pins that hold your front sight on should be removed, and set screws installed (4 each) to de-stress the barrel at that point.
5. Tighten up your sights. Whether optic or iron, accuracy lives in your sighting system. Most would be shocked to learn how much "rack grade" M16A2 rear sights move around.
Oh, well. I tried.