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"Pat No.: US D717, 906 SIncludes: Pistol Buffer Tube (PBT) Castle Nut End Plate Foam Pad