The new Hex-2 Pistol Buffer Tube from Phase 5 looks like a great option for those using the Sig SB-15 Not-A-Stock. It features a unique hexagonal shape near the receiver end of the tube that matches the contours of the SB-15 and serves as a stand-off to provide additional length of pull. It also utilizes a standard castle nut and end plate which means you have a host of different end plate sling mounting options. The Hex-2 comes with a nifty little cap that turns the empty space at the rear of the SB-15 into a storage compartment for ear plugs, a small bottle of oil, or maybe some Skittles.
Check out the new Hex-2 Pistol Buffer Tube from Phase 5.