Shotgun with pistol grip enlighten me

[chezuki]

If it was classified a pistol then why couldn't you make the barrel shorter than 18"? (Such as the ruger 10/22 charger)
seems I've been around guns for 30+ years and I wanna believe I know the answer to my questions, but I question the answers vs the law I think?

Because then it falls under the FEDERAL definition of an AOW.

Any Other Weapon
26 U.S.C. § 5845(e)
For the purposes of the National Firearms Act, the term "ANY OTHER WEAPON " means:
• Any weapon or device capable of being concealed on the person from which a shot can be discharged through the energy of an explosive;
• A pistol or revolver having a barrel with a smooth bore designed or redesigned to fire a fixed shotgun shell;
• Weapons with combination shotgun and rifle barrels 12 inches or more, less than 18 inches in length, from which only a single discharge can be made from either barrel without manual reloading; and
• Any such weapon which may be readily restored to fire.
Such term shall not include a pistol or a revolver having a rifled bore, or rifled bores, or weapons designed, made, or intended to be fired from the shoulder and not capable of firing fixed ammunition.