FN AR-15 Button Rifled Barrel 16 OAL M16, Carbine Length Gas System, 5.56x45mm. Made in the M16 styling, constructed of a mil-spec steel, and finished in black manganese phosphate, this 16 button rifled FN America barrel is a great choice for any carbine length AR-15 rifle. Chambered in .223 Rem / 5.56x45mm, the chrome lined chamber with its feed ramp will provide more consistent and reliable feeding while shooting.