Well a good start would be to ban future production (for the US market) and sales.
I doubt confiscation of existing weapons is feasible, but they should draw extra scrutiny when resold.
Whatever happens it needs to be federal.
I live in NY and we have a lot of gun laws. Some make sense, others are stupid, but a lot of them would restrict access to the guns that are being used elsewhere in these shootings.
Sure, a person can still find 30 round or 100 round mags, and not care about the consequences, but it's still a start.
I do worry about the slippery slope of gun control, but these "assault" style rifles attract weirdos in large numbers.