Hardened steel shims ensure precise fit and correct alignment of flash suppressors, muzzle brakes, and compensators. Large (thick) shim allows 90° of rotation and can be used as a spacer for barrels not having a relief cut behind the threads. Medium shims add approximately 45° of rotation. Small shims used with large and medium shims provide smaller degrees of rotation for finely-tuned, critical alignment.
AR-15, Issue:11, Page:022
Big Book Catalog, Issue:73, Page:031