Maybe start with that obvious little thing.. People not having guns means people not being shot at chicken restaurants, hence less crimes to solve. It does really work here in Europe even though it's not perfect - yes some criminals still have guns. But the big effect is someone being spotted on the street with a gun = insta-arrest + jail time for possession.

I know proposing it is political suicide there, but when I see what BLM have accomplished in awareness I did think some spontaneous movement like that would happen too in the gun debate, after the xx'th senseless mass shooting.

Also, the other obvious thing (also starting to bite Europe in the backside in all fairness), is the huge income equality leading to crime life.