And they've been running about 10 million "Form 4473"background checks for firearms transfers every year since then.
Of that 10 million, some are for transfers of "used" (already existing) firearms; some are for multiple purchases of new firearms. It's also my understanding that holders of Concealed Carry permits can buy without an additional background check.
The number of firearms in private hands is probably closer to 350 million.
Yep. And those firearms aren’t going ANYWHERE.