Here's some gun control I could get behind.
Our society uses felonies as a way to decide who gets to own guns.
One reason raising the age to buy a long gun to 21 would keep guns out of the hands of criminals is, it would give them a chance to get a felony as an adult before they committed a mass shooting.
Another thing which may help would be to put a five or ten year freeze on gun purchases for any person who committed a felony as a juvenile and was not judged fit to be tried as an adult. The freeze would extend into their adulthood. The intention would be the same. In that five or ten year period, they would show whether they were fit to own guns hopefully after maturing enough to not commit felonies.