This system will work with .223/5.56 x 45 mm NATO. - Kit includes: - Patented One-Piece Bolt Carrier - P Series Micro Block - Non-Adjustable (40% Lighter than the XLP!) - Upper Receiver Bushing - Bushing Installation Rod - Bolt Spring - Drive Rod with Bushing and Spring - Installation DVD - The Adams Arms Piston System: - A short stroke free floating piston system. - Is Self-Cleaning. - Carbon and gases are expelled forward away from operator. - Corrosive resistant coating processes. - Harmonics are not transferred into the barrel. - Doesn't vent directly to atmospheric pressure; this eliminates flash signature and super sonic crack which is optimal for night vision and suppresses fire. - Has a Lifetime Warranty. - This system requires a mid gas length (9.866 from barrel extension to shoulder) and a .750 gas seat. - We recommend using a standard barrel nut or barrel nut with similar measurements for proper fit and function of the kit. Many aftermarket proprietary barrel nuts do not offer the clearance for the drive rod or proper seating of the spring and bushing.