Sorry not able to answer original question. I'd equate your question to that of the masgazine law. One purchased in wyo X mags. The LE who chooses to look at your mags, has no clue where or when they were purchased. What does he / she do, besides walk away?
Question for you. How does / will anyone know what the end use of said ammo will be? Our daughter would buy ammo for our guns if she was where ammo was sold. The one time she was asked what the ammo was used in , she said rifle. That was it, paid for and left store.
I think (key word) you are over thinking the whole deal. A SMART shooter who is under X age should be accompanied by someone over 18 -21 when shooting a firearm, most LE's do not realize is legal to be in possession of. .