The terms you need to search are
NFA (national firearms act)
Short barrel shotgun
Since the shockwave only ships with a pistol grip (unable to be shoulder fired) it is legal to use with a barrel less than 18 inches in length.
But as soon as you install a buttstock on the gun, it becomes capable of being fired from the shoulder, and thus you would need to pay the $200 making/registration tax.
You could do it legally if you would like to pay the $200 tax, file the Form 1 paperwork, engrave the receiver or barrel with your name/city/state (because you effectively become the firearm manufacturer).
But, other than that, you would be committing a felony to install a buttstock.
Yes, it's sillly, and also why the nfa should be ruled unconstitutional for demanding a tax to exercise a God given right. But, this is where we are currently at. That is the ONLY way to be able legally to do it is through the extortion tax process. There are no weird tricks or ways around it.