Actually, a Shockwave is in the same ATF category as a pistol-grip-only shotgun and therefore it has the exact same legal limitations. Neither are legally shotguns since they weren't manufactured or transferred with a stock attached. And the federal definition of "shotgun" requires a stock:
https://www.atf.gov/firearms/firear...n-firearms-gun-control-act-definition-shotgun
Neither a PGO shotgun or a Shockwave have a barrel length limit since they're both not legally shotguns, they're non-NFA "firearms". The only limitation is they can't have an overall length lower than 26". They're both transferred as "other firearms" on question 16 of the 4473, the only difference between the two on the 4473 is that for question 27 you write "firearm with a pistol grip" for a PGO shotgun and just "firearm" for the Shockwave since the ATF doesn't consider a birds-head grip to be a pistol grip.
Yes, you can add a stock to a PGO shotgun that has an 18" barrel and make it legally a shotgun, but you could do the same thing to the Shockwave if you added an 18" barrel to it first. The only difference here is that a PGO shotgun usually comes from the factory with an 18" barrel already, whereas the Shockwave has a 14" barrel so you'd have to switch out the barrel before adding the stock.
Most of this is incorrect. Switching out the birds-head grip with a pistol grip makes it an AOW only because the firearm would now be below a 26" overall length. It has nothing to do with the pistol grip itself. The only reason the Shockwave comes with a bird's-head grip instead of a regular pistol grip is because a regular pistol grip won't get it over the 26" overall length limit.
Yes, adding a stock makes it a short-barrel shotgun if you leave the 14” barrel in place. But no, adding a vertical foregrip doesn't change its classification at all. Like I said before, the only time the ATF has deemed that a vertical foregrip changes the classification of a firearm is when you add it to a pistol. If you're adding a VFG to a pistol with an overall length less than 26" it becomes an NFA AOW. If you're adding a VFG to a pistol with an overall length greater than 26" it becomes a non-NFA "firearm" similar to the Shockwave.
https://johnpierceesq.com/can-you-add-a-vertical-fore-grip-to-an-ar-pistol/
Here are the only limitations to the Shockwave: You can't add a stock to it without first installing an 18" barrel, and you can't make it shorter than 26". Show me where the ATF says that adding a vertical foregrip to a non-NFA "firearm" makes it an AOW. You're not going to find it, and that's because the reason a VFG changes the classification of a pistol is because the federal definition of a pistol involves a firearm that's designed to be fired with one hand.
https://www.atf.gov/firearms/firear...on-firearms-gun-control-act-definition-pistol
The ATF has determined that a VFG changes a pistol's classification because it redesigns it to be fired with two hands:
https://www.atf.gov/firearms/docs/o...FjAAegQIDBAB&usg=AOvVaw1pWneRexYb6T7KC_VKk7bt
But since no other type of firearm is legally designed to be fired with one hand, a VFG doesn't change the classification of any firearm except a pistol. So it's fine to put one on a Shockwave.