If you wanted to be snarky sharky about it, the "bang stick" used to detonate a shotgun shell against a shark could be viewed as an NFA object;
Actualy they already are addressed, and they are viewed as such if they don't have a rod attached giving them an overall length of 18" or more. Most are sold with a soldered/welded rod "permanently attached" which is designed to be removed when they are attached to another shaft, such as a spear shaft.
Many divers do in fact remove the shaft altogether because they become much more convenient and portable, like a knife with some punch they can just store on thier belt in case.
I don't think most realize it becomes an illegal NFA weapon, not just something more convenient to dive with.
The most common ones are not those which use shotgun shells, but in fact several that use pistol calibers such as .357 magnum, .44 magnum, and .22s for hunting small fish.
I seem to recall one of the more effective common cartridges is the .223 remington because the quantity of gas created relative to the size of the cartridge is great.
Then the 12 gauge for use against larger fish. Blanks or live rounds can be used. Ironicly the blanks tend to do more damage because they have a higher charge in them, and it is the jet of water and gas that does most of the damage, not the bullet.
Most of those used by divers are homemade, smoothbore weapons with less than an 18" barrel, since the entire device is usualy only 6-8" long.
They are exempted with a rod attached that makes them longer than 18".
Some are used from spear guns, especialy the pistol cartridge ones, to launch an explosive tipped projectile for hunting or defensive purposes underwater.
Others are attached to a shaft meant to be held.
I have already seen the add for it (or something essentially the same) and it's at least in the prototype stage.
I'm betting that it will be such a monumental failure as a marketable product that no laws will be written to address it.
Yeah I saw it too, and thought it was the same thing.
I did a quick google and found a link to a link which linked to a magazine with this picture:
Which has a link to the main website:
http://www.waspinjection.com/
Which appears to be dead.
Maybe they went belly up (pun) maybe they renamed, or perhaps the newer company simply copied the idea and was the first to patent it, and then screwed the other guy over and threatened him with patent infringement.
I have seen it happen before. First guy never gets a patent, next guy does, and second guy has the legal rights to the design.
There is in fact even companies that do that for a living, horde and obtain patents they have no intention of using so others will be forced to purchase the rights. (I am sure it reduces innovation and production)
So I wouldn't just assume it wasn't successful. He may have in fact recieved a cease and desist.
Or it could be the same entity with a new name.