“rotating at many tens of thousands of RPMs when it comes back down”. sure about that?
You can expect a 5.56/.223 round to be rotating as much as 300,000rpm at the muzzle depending on barrel twist.
Fired straight up, the bullet will lose it's velocity and fall back to Earth. It will still be spinning mightily, however.
I can tell you more about my shooting experiences in the SoCal desert on a dust plain where I witnessed just this effect.