Let’s make them illegal, like drugs, because that works great.
The demand for guns in the US is high; if you don’t think this would become a lucrative black market you’re foolish.
The solution is more involved than just “regulate X”. Something is deeply fucked that isn’t going to be simply solved with a law, and could make things worse despite great intentions, just like prohibition did.
If the demand isn’t addressed, the problem will still exist. Same as prostitution and drugs.