You are right about one thing, gun control wouldn't inhibit criminals from acquiring guns. Laws mean nothing to these people.
BUT some criminals - those with felony records who cannot buy guns - DO purchase guns in private sales. In the USA, these sales are 100% LEGAL without any kind of background check, receipt, etc. A man can approach me in his prison orange, 5 minutes after getting out, and I can legally sell him a gun. Crazy, huh?
That is the gun show loophole that many are trying to change. Dudes buy an AR-15 for $1000, then walk around gun show with sign saying "$1500 private sale" and sell it to anyone who wants one.
Think about a person willing to pay that kind of markup - because they cannot get it legally!
That loophole is a bad one, and makes no sense. yet some scream about it.