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. Or, even better, eliminate the draft.
We did that. The United States military is a 100% all-volunteer force. No one is drafted ever anymore because of the response by the anti-war movement during the Vietnam War.