What Restriction on Guns Would Actually Make a Difference?

This thread is intended to consider what restrictions on guns (not other measures) would actually make a significant reduction in the number of people killed in mass shootings (not homicides as a general matter). We have plenty of threads about why we should or should not have more (or any) gun control. If possible (I doubt it!), I would like to hear what would it would take in terms of gun control take to reduce the victims of mass shootings. Frankly, I would suspect the opinions of gun owners to be more enlightening because this is really a technical question.

My two cents is that it would take something very significant: outlawing any semi-auto with a detachable magazine. The focus on the AR-15 or "assault rifles" veers into debates over things that are largely cosmetic. I don't see folding stocks killing anyone. But a relatively high rate of fire and the ability to reload quickly make a difference in how many targets can be hit in a short period of time. This is true with a rifle (e.g., Florida HS Orlando nightclub) or a pistol (e.g., Virginia Tech). This would, of course, outlaw the production or even existence (depending on the nature of the restriction) of many of the most popular guns in the USA. It would also do very little to reduce the overall murder rate because most murders - or suicides - are easily accomplished with a revolver or other weapon that would be unaffected.

What say the board?