every stripped lower reciever is classified as a pistol unless the person buying is under 21 in which case by law they can only sell rifles and sell them as such. nobody is going to blink twice at an AR in pistol configuration.
That may be true in other countries, but in the USA, a receiver is classified as a receiver. Not a handgun and not a rifle or shotgun. Just a receiver. State laws may refine the sale, but federal law does not make a configuration distinction.
Please consult the BATF website for more information.