I've heard figures as high as 900 million, but who really knows? As this article pointed out, the U.S. government is forbidden from tracking this information.
But this article also refers to "advances in military technologies that rendered personal firearms all but useless against professional armies." Do you think that's true? That's also what Joe Biden once claimed; what can citizens do against F-16s and nukes?
https://www.timesofisrael.com/comparing-america-to-israel-on-gun-laws-is-dishonest-and-revealing/
