BudsGunShop.com - Classification of this lower

Gary, no, Attaching an AR-15 pistol upper assembly (which is an upper with a barrel SHORTER than 16″) to a lower with a fixed or collapsible stock is also considered an SBR and will get you in trouble without the proper tax stamps in place. If one is putting together an AR-15 pistol, one cannot modify a rifle to a pistol. This has a shoulder stock and is classified as a rifle, so it cannot be made into a pistol. Conversely, if you have an AR pistol and later modify it to a rifle, and then back again to a pistol, this is legal.

Gary, looks like this is classified as a rifle and has a shoulder stock on it. So unfortunately you can't use this as a pistol lower. Sorry but hope this helps