We can't prove that, and we should be cautious of using it as an argument.
Let's say Town A has less restrictive gun laws, and they witness a drop in violent crime. It could be that they put more cops on the streets, that their economy improved, or that they've got a judge who's a stickler for minimum sentencing.
While it's easy and tempting to
infer a causal relationship, we have to be able to
prove it if we're to use it as a debate point, and that's hard to do.