Just going by NICS background checks from 1998-2018,there were 280,482,910 background checks performed.

Not every check was for just one firearm,as you can buy multiple firearms using a single background check.

NICS checks-19998-2018

How many millions of guns were sold prior to NICS being implemented?

Just using modern firearms -(after metallic cartridges replaced black powder muzzleloaders)-as a guess,starting in late 1800’s with guns like say the Winchester ’92 and ’94,and Colt revolvers,there were likely at least another roughly 300 million firearms-or more- manufactured and sold in the approximately 100 years between modern firearms being mass produced and the NICS check being implemented.

The late WeaponsMan did a post on this topic here

From that post-

“We now have 329 million firearms, with fairly trustworthy data and estimates in which all the most conservative assumptions were used, introduced to the US on-the-books market from 1999-2016.”

“Absent a better idea, we can say that the US inventory of firearms is almost certainly between 412 and 660 million, not the lower numbers recently trumpeted in the media.”

Another blogger-

Extrano’s Alley

addressed the topic here

“Between the number of surving guns from the 19th century, production and d survival numbers from the 20th Century, and manufacturing and import numbers for the 21st Century we should have between 710 and 770 million guns that are either serviceable, or easily restored to service. That is, guns that have no broken parts or serious internal rust problems, needing only a clean and lube to restore to working order.”

“On that basis, we ESTIMATE there are 740 million firearms that are A.) Serviceable as is: B.) or easily restored to service. This does not include firearms in caliber such as .41 Rimfire for which ammunition is essentially unavailable.”

Another thing to consider is that there has been an increase in home manufacturing using 80% lowers for A-R’s,and 80% receivers for handguns like the 1911,plus you can now 3D print a functional firearm.

Then there’s the modern blackpowder firearms which are not included in BATFEIEIO data,as they do not require a NICS background check.

As machine shops upgraded/upgrade to CNC machines,there have been hundreds of thousands if not millions of used,fully functional lathes and milling machines sold to individuals. Many of the guys-and ladies- who bought those machines are gun owners.

Not much you can’t make when you own a lathe and milling machine,if you own those,you likely have the rest of the tools to make pretty much anything you want or need.

 

Taking all the methods of home manufacture mentioned above,along with the plethora of ways to make your own functional single shot firearm with basic hand tools,my own scientific wild assed guess is there are somewhere north of 750-850 million firearms owned by citizens in the U.S. (Thanks to chulai1968 for catching my mistake-I typed 100 million originally)