AR-15 Pistol Buffer Tube (PBT)(TM) by Phase 5 Weapon Systems Inc features our billet aluminum buffer tube, foam pad constructed of ultra-durable nitrile rubber, plus Mil-Spec end plate and castle nut. Buffer tube is compatible with Mil-Spec carbine buffer and spring, and the lower slot is machined to accept all Mil-Spec end plates and sling mounting accessories.Outside diameter: 1.25"Includes:Pistol Buffer Tube (PBT)Castle NutEnd PlateFoam PadCarbine Buffer - Standard 3ozPhase 5 Enhanced Buffer Spring