Since the BATFE has defined a pistol as having a rifled bore there is no way a shotgun could legally be considered a pistol.
The only firearm categories which a shotgun could be considered would be :
Shotgun â€“ barrel no less than 18 inches, overall length no less than 26 inches, with or without stock.
NFA â€“the following three items must be registered under the NFA
Short Barrel Shotgun â€“ barrel less than 18 inches, overall length not an issue, as a short barrel shotgun. $200 making tax to make from a shotgun. $200 transfer tax.
Any Other Weapon â€“ barrel less than 18 inches, cannot have a buttstock. Cannot be made from a shotgun â€“ must start at the factory as an AOW. $5 transfer tax.
Destructive Device â€“ Barrel length irrelevant. Generally the BATF declared these to be DDâ€™s by make and product name. The Street Sweeper is one example. $200 transfer tax.