Why would there be less guns on the street? Aren't the vast majority of those illegally obtained anyways? IMO the more likely response would be that all the guns that were previously stolen/criminally obtained legal guns would become fully automatic since there's no reason not to at that point. (Manufacturing cost difference is basically nil, and if you're already smuggling an illegal rifle might as well make it automatic)

Guns aren't exactly rocket science to make, and the plans are available online. Good guns, reliable guns, safe guns? Not so much, but why would the criminals in question care?