Somebody needs to construct a Venn Diagram of the various factors and decide whether or not a simple banning of the weapon is an effective remedy.
Let's see.
Young kid, (under 35?)
Able to purchase and possess several hundred rounds of ammunition, able to have delivered to their residence.
Able to purchase high capacity clips, clips which have zero tracebility (so you can't shame a straw party purchaser).
Able to purchase and possess multiple handguns, and multiple long guns.
This would apply to Charles Whitman, the Columbine shooters, the Aurora shooter, the Newtown shooter, etc.
We prohibit anyone under 35 years of age as President, because of the great and grave responsibility, right?
Why not craft a law that prohibits this apparently deadly combination of youth, high capacity magazines, multiple semi-automatic weapons and huge quantities of ammunition?
For example, anyone under 35 can possess a maximum of one semi-automatic weapon, either long gun or hand gun, but only one.
Serialize the high capacity magazines for tracing (and shaming) anyone who sells or buys a high capacity magazine that ends up at a crime scene.
As far as entry to schools, $60,000 will have Hamilton Safe come out and install an "airlock" system integrated with a metal detector; you set off the detector, you can't get in the bank (or school).