The vast majority of guns used by the Mexican drug cartels are surplus, military weapons coming up through South and Central America. They are fully automatic, actual military weapons (Class III like another poster said). They are not legally purchased, semi-auto rifle or handguns from the U.S.
This is pure disinformation, and propaganda being spewed by Obama, and Holder to justify disarming the U.S. law abiding populace. That is why they ran Fast and Furious, so they could point to guns obtained in the U.S. and ban them.