VooDoo Gas Tube - VDI gas tubes are compatible with mil-spec and non mil-spec upper receivers. This gas tube is machined to exceed all MIL TDP specifications and is enhanced using a proprietary LifeCoat treatment. This is a process that offers superior: - Durability - Lubricity - Corrosion Resistance - Requires a barrel with a mid length gas system - measures approximately 9.866 from barrel extension to shoulder.