I agree that it is not 100% proven that banning guns would reduce gun crime but it doesn't seem like much of a stretch that it might reduce it just a little bit. I do agree that only banning assault rifles is pretty much pointless.
I don't think no guarantee of ROI is a good reason not to do it. Sometimes you have to take chances and what I am proposing would be a tiny fraction of the budget. I mean less than a tenth of a percent. I think the work should be done primarily by private entities and universities, I am mostly talking about funding. If they were already making significant progress without government intervention I would say that is best but I'm not sure much of that is going on.