From the ATF FTB letter to Mossberg 02 Mar 2017:
p.2 "The weapon, as described above, is a "firearm" subject to GCA provisions; however, it is not a "firearm" as defined by the NFA. Please note that if the subject firearm is concealed on a person, the classification with regard to the NFA may change."
And in ref to the exact sample submitted for ATF Firearms Technology Branch approval:
p.4 "The Mossberg, model 590, serial number V0348718 as described above, is a "firearm" subject to GCA provisions; however, it is not a "firearm" as defined by the NFA. Please note that if the subject firearm is concealed on a person, the classification with regard to the NFA may change."
Legality of the Shockwave may also vary under state or local law, and there are limitations on use, where, when and how, even in jurisdictions where the gun is legal.