not true. a family member in virginia has a cannon. shoots it off every Independence Day. of course the cops always show up. but no law was broken. been doing it for decades.
I have a 1 1/8” Coehorn mortar that I shoot off on the 4th as with a salute charge.
Took it out to a lake once and put a rough cast lead shot in it. Was impressed at the 250 yrd range with only a moderate charge of FF in it.