I am assuming this count does not include private sales.
I suspect the bulk of those numbers represent new-manufacture firearms moving “into the inventory”.
Private sales all represent existing firearms; they do not increase the overall inventory.
No, this is based off of background check data for gun purchases, and reasonable assumptions on how many of those checks resulted in an actual purchase. Private party sales are an unknown addition to these figures.
And despite not all purchased guns being of an "assault" nature, the average deer/hunting rifle has an effective range out to 300~500+ yards depending on the user. Ammo that drops a deer will easily penetrate body armor - and there are millions of active deer hunters across the US based on annual deer permit issuances. Hence, the oft referred to term here of "a million man deer rifle army."