The number from that chart is a flawed number. A buyer at a gun shop or an auction can buy unlimited long guns and the #4473 form is only counted as 1 background check. And a 4473 doesn’t cover black powder firearms.
True, but buying used guns does not add to the overall stock, and NICS checks are also done for CCW licenses.
My best guess is that we are at about 320 million firearms in the U.S. now. Perhaps a range of 280 to 340 million would be a better way to say it.