The reason for the shockwave is to legally have a barrel under the legal 18" law and. The shockwave has a 14" barrel but it's still 26" in overall length so it's legal as a "firearm"
Right. The second you add a true stock to the Shockwave (which you most certainly can do), congratulations, you've created an NFA regulated SBS, and now you have to pay for an additional $200 tax stamp and run a bunch of paperwork on it. The arm brace doesn't qualify as a "stock" under the law, and no one is ever going to question if you shoulder-fired it in a home defense situation.