DoubleStar M4 6-Position Modular Stock w/o Boss Black. This stock features a "squared hole" design to allow sand and grit to pass through the tube area. Specifications for DoubleStar M4 6-Position Modular Stock: Manufacturer: DoubleStar Color: Black Gun Model: M16 M4 AR-15 Stock/Brace Material: Plastic Gun Type: Rifle Weight: 13.5 oz Length: 9.75 in Gun Make: AR Platform Fabric/Material: Plastic Features of DoubleStar M4 6-Position Modular Stock For use with FSM FSM-PB and all Ace receiver blocks Package Contents: Doublestar M4 6-Position Modular Stock What firearm models does the manufacturer say DoubleStar M4 6-Position Modular Stock is compatible with? The manufacturer says that DoubleStar M4 6-Position Modular Stock is compatible with AR-15 M4 M16DoubleStar A311: M4 6-Position Modular Stock w/o Boss Black