[dakota.potts]

There is no requirement that the stock can't be capable of physically attaching to the gun when the pistol barrel is installed.

Only that all parts are installed in a legal order and have a legal use. You can't have a stock laying around if you don't have a rifle barrel, but you can have a pistol, and a stock that can attach to the pistol, as long as you also have a rifle barrel and you only attach the stock after attaching the rifle barrel.