USGI and many commercial barrel nuts require some time of timing in order to either get the gas tube into the upper receiver or line up a set of screws. In many instances, the ideal torque spec of 40-60ft LBS is much higher than is desired for an accurate weapon. One of the few ways to control the torque value of a barrel nut is with shims.

These shims are a great way for the consumer to set the exact torque spec their barrel nut.