That wouldn't work because people who go on mass shooting sprees either get them illegally (because there's so many guns in the country and they can scratch the serial number off) or steal them from family or others.
The issue is the fact that there's so many around and civs have them. There's always going to be mass shootings as long as that's happening. So it comes down to a choice regarding having less guns to circulate so they can confiscate them over time and focus on the grey/black market, or continuing to manufacture and sell them to people.
So unfortunately most people are going to say "I want to keep my guns", and it's going to keep happening because gun laws actually don't work that well as some have pointed out.
