I bought one years ago and have never done anything with it.In fact I am not 100% sure how it works.
Its a 12ga 'sea hornet' brand in two pieces. One looks like the barrel/chamber- just a metal cylinder.
The other bit has the spring loaded firing pin poking through the 'bolt face' I assume. And a threaded bit on the end.

Would this one have a shaft screwed in for hand use, or do these connect to a nromal spear gun somehow?

Firgured I might as well get round to using it on something-probably a watermelon test in a creek

karl.

Anything like a "bang stick" used by scuba divers to punish the sharks?

That's them! And they can be fixed to the end of the speargun for use also, I just found out.
The dive shop lady I talked to said people use them on land as well! for who knows what?
Imagine the recoil of a 2 lb 12 ga


Karl.

These units if carried or used in the wrong manner are considered a destructive device the same as a sawed off shutgun. At least in California. They are really cool though. I have seen them in calibers such as 223,357,44 mag and the 12 gauge