In 2018, the late Kevin O'Brien (Msg, US Army, Special Forces, Airborne Ranger, Retired), AKA Hognose, former owner/operator of WeaponsMan.com, conducted what I consider the most comprehensive
and impartial survey of weapons in civilian possession yet conducted. His tally was a minimum of 412 million and (owing to sloppiness inherent in the records-keeping system) possibly as many as 660 million.
So considering the brisk sales since 2018, the 600 million number could be close ... or it could be 250 million short.
https://weaponsman.com/total-us-firearms-not-300-million-but-412-660-million/