These thin metal nut washers should always take into account when you want to create a tight and precise rail installation, as they serve as useful devices to evenly space the rails and barrels with the receiver, as well as minimizing the play in between.
Be cautious not to put too many shims as that will prevent the rails and the barrels from tightening perfectly with the receiver, while not enough shims will cause too much points of contact. So you know how to make it work.....trial, trial, and trial!!
Package comes with different sizes: 2mm/1mm/0.15mm/0.1mm