So how, exactly, do you know that all of these ammo-buyers are what
you call "law-abiding citizens"?

What's your point?

My point, if one were too dense to deduce it from my question, is this:
how many of those 7.5 billion rounds would end up in the hands of those
you'd rather it not (like people who might want to rip you off or
otherwise do you harm)?

Well," if one were too dense" to deduce the flaw in your question,
hence my question, the I'll have to explain.

1. For every box of ammo sold to each non law abiding citizen I would
hope at least one or more boxes would be sold to each law abiding
citizen.

2. If you are implying sales of ammo should be limited because a non
law abiding citizen may purchase some then you must also believe that
sales of cars should be limited. For some non law abiding citizen may
use it as a get away car.