Please post a source... Id like to read the specifics and who claims it to be such. Im not saying its not true but I have never seen a credible source on it.
My source is common sense. Someone with a criminal record cannot buy a gun. I'm assuming, of course, that NICS denies them. I'm sure a small number were purchased legally by the criminal (perhaps before they had a criminal record), Or maybe due to a fault in NICS, but the vast majority, common sense dictates, do not. Keep in mind that a person to person sale to a felon is still an illegal sale, whether the seller knew it or not (though it is a legal grey area whether he'll get in trouble if he didn't know).
As for a credible source, I do not have one, and honestly, don't feel like looking. I know I've read secondary sources that mention it, but never seen actual statistics. I'm just curious, how else could a criminal (assuming NICS is 100%, which we know it's probably not, and I already said a small number could have been done this way) get a gun if it wasn't purchased, or stolen from a law abiding citizen? Smuggled in from Mexico I suppose? Though most guns go South...not North.
EDIT: I also want to make clear, I do not believe in mandating people to keep guns in a safe. That just makes gun ownership more of a burden, especially for those that might need one, and don't have a whole lot of money. All I'm trying to say is, secure your weapons as much as you reasonable can, within your means. I would be adamantly against any laws that mandated otherwise.