tahunua001- I think this one has been beat to death in endless forum topics.
If it's a new lower that's never been a rifle, it can be a pistol or a rifle.
Once it's been a rifle, no more chances to be a pistol. Even if you built it as a pistol, then added a 16"+ barrel and a stock to make it a rifle- there's no reverting back to pistol configuration.
Like FALPhil said, it's pretty black and white.