Christ! You do know most states have laws banning the possession or construction of zip guns? It’s a bit hair-brained to call them “perfectly legal”, either that or just plain entrapment on your part.
And, no, it doesn’t sound like he was making a zip gun. I’ve seen examples of booby-traps recovered from raided grow-ops that were constructed as generally outlined in the articles that describe this arrest. The devices I’m thinking of do very decidedly meet the NFA’s definition of a DD.
To my knowledge the slam-bang is legal if it does not violate other laws, such as minimum barrel length, and that the builder submits the proper federal forms.
I may be mistaken though.
If you know more about it and can give some specifics instead of casting aspersions it would be helpful.