Greed
Everybody says criminals steal their guns. And yes they do that too. But what is being overlooked here is the law of supply and demand, profit margin, and just greed.
What I mean by that is the market that Mayor Bloomberg is so misguidedly trying to crack. The purchase of guns in an area where they are legal, and their resale (for tremendous profit) in an area where they are not.
Now, under our current laws, this is a crime. actually, I think more than one. But the guns are not stolen, they are paid for, legally purchased as far as the seller knows.
If you can buy a cheap pistol for under $100, drive six or seven hours (or less), and sell it for $4-600 ( I have no idea the cost of blackmarket guns, but I hear they are expensive), time after time, some people are going to do just that. Even if they do break a few laws. Like drugs the tremendous profit ensures some people do it no matter what.