No, he didn't. He used a bump stock, which is not illegal. And I don't mean "obscure and unregulated," I mean the ATF has specifically approved their sale and use as not changing the function of a firearm from semi automatic to fully automatic.

Bump firing isn't dependent on a $400 gimmick either, it's a simple technique that utilizes the recoil to facilitate the next trigger pull. It can be done with a pen, a shoelace, a belt loop, or basically anything.

There has never, to my knowledge, been a shooting that used an illegally modified fully automatic firearm. There have been two, that I'm aware of, actual full auto weapons used in crimes within something like an 80 year timespan.

As to preventing mass shootings, deterrence, enforcement, and response.

Deterrence being the removal of gun free zones, advertising the fact that teachers and staff can be armed, effective limiting of access to school grounds, national reciprocity of conceal carry licenses, a national level push for effective and economical training for willing carriers, and informing both a potential threat and their potential targets that the targets have been alerted to the possibility of a threat if red flags appear.

Enforcement being actually enforcing the laws as they stand, funding NICS, tie federal aid to state compliance with NICS reporting requirements, and identification and enforcement of prohibited possessor laws.

Response being an actual, credible, effective response in the event of an active shooter.

There's dozens of things that can be done that don't trample on second amendment rights.