This is what 2A Armament says:
This 3 piece AR10 buffer tube shim kit is designed for builders to address the non-standard AR10 market. These shim provide adjustment in bolt carrier travel and can also be used to avoid potential interference and impact of an AR10 bolt carrier with the lower receiver. Three different thickness shims are provided (.070, .100, and .140) that can used individually, or in any combination to obtain a wide variety of shim thickness, from .070 up to .310. These shims should be placed in the buffer tube, prior to installing the buffer spring and buffer. It is important to verify clearances, and function of bolt carrier, including lock back when using these shims.