I have used the red wedges before. They serve a purpose. There can be enough tolerance stacking between uppers and lowers of different manufacturers that a little movement exists. I've had it between uppers and lowers of known good brands even. The best solution I've found is to use lowers that have an adjustment screw under the grip in the lower to snug up to the rear takedown lug. I've been using Centurion lowers for my last several builds and they have it.