Reality is, it's NFA and if a metal tube falls off the bang stick, you'd have the entire battalion combing the area where it was lost until it was found.
Even if it's dropped to "long gun" status, military's not going to touch it. Whenever it comes legally on par with magazines (ie if one fell in the mud and someone accidentally picked it up, no felonies would be committed), you might see the military look into it. Obviously, special operations units use the hell out of them because they can use common sense and try to reduce hearing damage.