FN AR-15 Button Rifled Barrel 16 OAL M16, Mid Length Gas System, 5.56x45mm. Styled to reflect the M16 platform and made from strong mil-spec steel with black manganese phosphate finishing, this 16 button rifled FN America barrel is an ideal choice for any mid 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.