"According to the 1997 Survey of State Prison Inmates in the US, among those possessing a gun, the source of the gun was a flea market or gun show for fewer than 2%, a retail store or pawnshop for about 12%;
family, friends, a street buy, or an illegal source for 80%."
http://www.ojp.usdoj.gov/bjs/guns.htm
US Dept. of Justice, National Crime Victimization Survey, 2000
This means that for 80% of criminals, anti-gun laws have no effect at all. Obviously, though, they disarm 100% of law-abiding citizens.
Let's be absurdly optimistic and say that none of the 20% of criminals who obtain guns legally are resourceful enough to get them illegally, so banning all guns stops 20% of gun crime. 20% of 12,000 is 2400 -- versus 100% of the 1.5 million crimes that are committed because law-abiding citizens no longer have guns to stop them.
This is why I support gun rights. It makes me, and you, and all of us, less likely to be injured or killed by a criminal.
This holds true whether you own guns yourself or not, so even if you dislike guns yourself and would never own one, you should support gun rights.