Dylan and Kliebold weren’t. Neither were several others.
But Dylan and Harris were underage and did not buy the weapons themselves anyway. They had a friend who was 18 buy them.
“Because Harris and Klebold were both underage at the time, Robyn Anderson (with whom Klebold attended the prom three days before the shooting), an 18-year-old Columbine student and old friend of Klebold’s, made a straw purchase of two shotguns and a Hi-Point carbine for the pair”.
So in that case Feinstein’s law would not work (if passed) and would lead to the same sort of thing among others if it does become law. The ones wishing to use the weapons for murder would find a way, as they have in the past.