Many of today's aftermarket AR-15 barrels include the barrel nuts required for installation, but there are some barrels sold that rely on mil-spec barrel nuts, and you may not get a barrel nut with the barrel. No worries, though! CMMG has you covered with this mil-spec barrel nut designed to work with mil-spec barrels and upper receivers. The nut is machined from steel and features an all-black finish.