Ok- for the record:
If one could locate a receiver that has not been assembled as a shotgun, it could be registered as an AOW, but the initial tax would still be $200. Subsequent transfers would be $5.
If you make a pistol from an existing shotgun it can not be registered as an AOW- it would be a "firearm made from a shotgun" with an initial and subsequent taxes of $200.
Therefore, the gun could be either an AOW, or a "Firearm," but not technically a SBS (they have a stock) unless rulings have been changed recently.
Why would anyone want a rifled 20 ga. pistol? Rifling was not mentioned.