40 percent ? Where does that number come from ?

Given many unfortunate events as those who have taken place in Boston and Connecticut, the craze for gun control has gone wild around the U.S. While writing a paper for my public policy class I couldn't help to notice that President Obama and Biden have been stating that  "40 percent of guns are purchased without a background check.” I am not against gun control, or more extensive background checks, but the use of the number intrigued me.

The purpose of using this number, relies on the idea that there are many ways to avoid traditional background checks, yet, the number is quite hard to estimate.
The Brady Handgun Violence prevention Act currently prevents someone who buys from a federally licensed dealer from buying a gun if he has a felony, some kinds of misdemeanors conviction or has been found mentally il, yet, a loophole in the act allows individuals to sell their guns privately, and thus, opening the possibility for background checks.

 Trying to find the source of that 40% I have gone into a crusade that gives me not reliable answer. Many different researches quote different numbers, but most importantly, the sources tend to be really biased groups from both sides of the debate.

This experience taught me, that numbers can be as dangerous as guns when dealing with people

-Xavi