Heroine and cocaine are illegal but sales are great. Marijuana is still illegal in some states yet business is booming. Remember prohibition? Outlawing alcohol only led to a huge volume of black market sales with providers making outrageous profits because they jacked up the prices. The bad guys like Capone wanted alcohol to be outlawed.
The bottom line is anyone saying they're going to fix a problem by making it illegal, doesn't have the data to prove it. I've said it before, I don't mind outlawing certain weapons, but what I'm saying is that the politicians who are trying to gain praise by saying they'll "fix," the problem through gun control, know they won't.