Gun shows account for 1.9% of guns used in crimes according to the FBI (and much of those were transferred via a NICS check through an FFL).
I don't know what percentage non-gun-show private sales make, but it would appear that closing the "gunshow loophole" (at least as far as gunshows are concerned) would do very little at all to prevent guns getting into the hands of criminals.
I don't know what percentage non-gun-show private sales make, but it would appear that closing the "gunshow loophole" (at least as far as gunshows are concerned) would do very little at all to prevent guns getting into the hands of criminals.