I'm trying to find out. I got this so far:
Is gun theft a part of the illegal gun problem?
Yes, gun theft is one of the ways guns are diverted to the illegal market. The ATF reports that over 13,000 guns are lost or stolen from dealers nationwide each year. Estimates show that as many as 32% of crime guns are acquired through theft.5 6
www.mayorsagainstillegalguns.org/html/about/faq.shtml
( Moon Girl Technologies don't take my word for anything, Wed 9 Jan 2013, 16:57, Reply)

"According to ATF data, 70 percent of firearms recovered and traced in drug cartel crimes in Mexico originated from the United States."
www.politifact.com/virginia/statements/2012/may/29/jim-moran/jim-moran-says-70-percent-traced-firearms-mexican-/
( Moon Girl Technologies don't take my word for anything, Wed 9 Jan 2013, 17:12, Reply)