Play between upper and lower is the norm, and in theory has no effect on accuracy. All the zero and mechanics except the trigger is contained in the upper itself. There for your sights dont lie. As long as you are on target, the gun cycles as it was meant to (BCG recoil cycle doesn't bind up), and the hammer can reach the firing pin, it will not effect accuracy no matter how much play there is. However, thats not to say that a large amount cant effect your aim, but it would have to be a lot and you would have to be applying an inconsistent pressure during shots (physically trying to wiggle it) to actually have an issue.