@Dane you know I went and looked it up and it turns out we were both wrong. The “non-expanding bullets” rule is addressed in both the Geneva and Hague conventions (plural). And while the U.S. did not sign one of the Hague conventions they did sign the others.
I know its a nerd thing but I always try to make sure I do not post erroneous info. a big thank you to Dane for pointing me in the right direction.