What Rick said. The barrel length on the shockwave-style shotguns is under 18 inches. So it's a felony because it doesn't include "intended to be fired from the shoulder" or "having a stock" or anything similar in the definition like under the NFA. Under that definition, the Taurus Judge is illegal too. Even all 9mm, 40, and 45 pistols would be illegal if the snakeshot you can buy at walmart "may reasonably be expected to be able to cause lethal injury." Under the NFA, shotguns are defined to include "intended to be fired from the shoulder," which the shockwave-style shotguns clearly aren't. See 26 USC 5845.
21 OS 1289.18:
http://www.oscn.net/applications/oscn/DeliverDocument.asp?CiteID=69775
A. "Sawed-off shotgun" shall mean any firearm capable of discharging a series of projectiles of any material which may reasonably be expected to be able to cause lethal injury, with a barrel or barrels less than eighteen (18) inches in length, and using either gunpowder, gas or any means of rocket propulsion.
B. "Sawed-off rifle" shall mean any rifle having a barrel or barrels of less than sixteen (16) inches in length or any weapon made from a rifle (whether by alteration, modification, or otherwise) if such a weapon as modified has an overall length of less than twenty-six (26) inches in length, including the stock portion.
C. Every person who has in his possession or under his immediate control a sawed-off shotgun or a sawed-off rifle, whether concealed or not, shall upon conviction be guilty of a felony for the possession of such device, and shall be punishable by a fine not to exceed One Thousand Dollars ($1,000.00), or imprisonment in the State Penitentiary for a period not to exceed two (2) years, or both such fine and imprisonment.
D. It is a defense to prosecution under this section, if the approved application form that authorized the making or transfer of the particular firearm to the defendant, which indicates the registration of the firearm to said defendant pursuant to the National Firearm's Act, is introduced.
26 USC 5845:
https://www.law.cornell.edu/uscode/text/26/5845
(d)Shotgun
The term “shotgun†means a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of the explosive in a fixed shotgun shell to fire through a smooth bore either a number of projectiles (ball shot) or a single projectile for each pull of the trigger, and shall include any such weapon which may be readily restored to fire a fixed shotgun shell.
