Fits into the end of the bolt carrier key to ensure that the barrel nut is properly aligned during installation of AR-15/M16 barrel. Improper alignment can damage the bolt carrier key and/or the gas tube.
AR-15, Issue:09, Page:076
Big Book Catalog, Issue:66, Page:074
SPECS: Steel. .178" (4.5mm) diameter x 3" (7.6cm) long.