Disclaimer: I own a few guns and am an Army vet, but this is just my current understanding. I recommend consulting the ATF guidelines yourself and/or your local sheriff's office since rules tend to vary by state and jurisdiction.

The ATF makes these rules, and currently (my understanding, haven't looked in a while to see if anything changed with the new administration) an AR pistol is something that's less than 26" overall with a barrel of less than 16". There's also rules about how you can't put an actual buttstock on it (but you can put a "brace" ... the definition of which can be confusing). Anything shorter than that and does have an actual stuck is an SBR (short-barrel rifle) and you need an ATF tax stamp for that.

I appreciate your time to explain that, thanks. I'm now off on a journey to find the exact line between what is a brace and what is a stock! Take care, my friend.