This kit contains all the necessary parts to mount your AR15, M16, M4 Pistol Lower Receiver. Pistol receiver extension (buffer tube) Machined from 7075T6. Hard Coat Anodized per Mil-A-8625F, Type III, Class 2 . 1.2 outside diameter. Will fit your ATF compliant SigTac SB15 type pistol stabilizing brace. Works with carbine buffer and action springs. BCM(R)(TM) QD Receiver End Plate Lock Nut (Castle) M4 Action Spring, Carbine Buffer Assembly, Carbine (options for H, H2,H3) This kit is compatible with your ATF compliant SigTac SB15 type pistol stabilizing brace.