As you say very few guns used in crimes are legal.93% of guns used in gun related crimes are illegal.
Just because something is illegal, doesn't mean you can't obtain it. Sure I would say RPG's and grenades are not easy to get hold of illegally; but guns are. I'm really in fear of repeating myself here and its honestly getting boring, but you can quite easily obtain an illegal firearm if you wanted to pursue that. That's the bottom line really.
However where murders are concerned there are two key exceptions - domestic violence or mass shootings. You are assuming that there will be a 100% replacement of these legal guns with illegal guns and so no reduction in the acts - I don't find that credible.
http://www.politifact.com/punditfac...-joe-scarborough-tiny-fraction-crimes-commit/
"I think it’s safe to say that a low percentage of criminal assaults and robberies are committed with guns that were acquired by legal purchase from a gun store," Cook said. Joseph Olson, a professor at Hamline University School of Law and former board member of the National Rifle Association, believes that the number of legally acquired firearms used by criminals is negligible."Criminals don’t go through background checks because they know they wouldn't pass them," Olson said.
Olson said with homicides, there are two key exceptions. Legal firearms are often found when the killings occur duing domestic violence or mass shootings.