These wedges are designed to push an AR upper receiver upwards to remove slack that can cause the upper to wiggle and lose accuracy during firing.  It is made from a syntheticr ubber and can be sanded/trimmed to fit if needed.  Many times it drops in without any adjustment being needed.

1/5/21 Update:  I'm getting more made and should have them the week of 1/18/21.  I'm turning back orders on and if you see the inventory go to zero or negative then know that I will start shipping that week.