You want me, someone who works in Higher Education and doesn't hold a public office, to prove that legislation that hasn't been enacted yet will work? Exactly how do you propose that I do that? So, I'm not allowed to have an opinion on the subject because I can't prove that theoretical legislation would work? Is that what you're saying?
Part of what people are saying is that the guns are already out there. Registered, unregistered, stolen, traded on the black market, etc.
Institute national registration with draconian teeth right now, and the situation is little changed. I do think that some effect on general gun violence can be had -- some "crime of passion" and "I'm PO'd right now!" deaths can be curtailed. But the Columbine-type events are very different, with long-developing plans and plenty of forethought. In a sense, guns are incidental to the damage done. Where guns cannot be gotten, the weapon of choice can morph into pipe bombs, or pressure-cooker bombs, or the like.