The Phase 5 Hex-2 AR-15 Pistol Buffer Tube Assembly starts with a hexagonal shape and the rear most portion is round to accept the foam pad. The Hex-2 is a heavy-duty USA certified 7075 T7 billet aluminum buffer tube with the versatility to use the 3.5" Phase 5 nitrile Foam Pad (included). This design, based on the recommendations from their customers, makes the Phase 5 Hex-2 Buffer Tube is a perfect component for any AR build. FeaturesHex-2 Buffer TubeCastle NutEnd PlateFoam PadCarbine Buffer - Standard 3ozPhase 5 Enhanced Buffer Spring