The AR-15 "Accu-Wedge" is designed to reduce play between the upper and lower receiver. Some higher end manufacturers have started to incorporate a "set screw" into their lowers that achieves this same effect. Less wiggle = more accuracy.
Made with a modified version of thing:635723 on Thingiverse.
Credit goes to user PrintedFirearms
Made with a modified version of thing:635723 on Thingiverse.
Credit goes to user PrintedFirearms