Illegal, BATF would classify it as a suppressor.
There you go guys, that'd be perfect for them. Necessitate black powder, which has more particulate. Since it has more particulate, firearm needs a particulate filter. Once it has a particulate filter, it has a 'suppressor', which is illegal. Since all guns would be illegal, 'no one' will have guns. We'd need to hire a bunch of environmental cleanup crews though, because the amount of Lefties that logic would satisfy would be so many, they'd all cream their pants in unison ready it. Jizzathol? Could be onto something there...