Maybe the focus on ineffective broad proposals is the problem. Raising the age to own a firearm to 21 would be more focused and I think legally defensible.
the inevitable argument would be “whoa, I’m old enough to be drafted to carry a weapon in a war, but not old enough to buy one myself???”
Raise the draft age.
You’d have to do more than that, you’d also have raise the age at which one can willingly join the armed forces
Or, even better, eliminate the draft.
it is needed for emergencies.