Actually the Mexican Army has never liked having US made weapons. Currently they are converting over from license built H&K 91 (G-3) to a locally produced derivative of the H&K G-36, the FX-05 Xiuhcoatl
However some of their police use the M-16/AR-15. But I suspect most of those in the hands of the cartel came from non US produces, such as China
Perhaps without the normal Norinco markings
There are other countries which build the M-16 or clones of it, some under license, some not. Iran builds a version, but the issue furniture makes it not look like a US M-16.
I stand corrected, I based my supposition on photos published by the MSM of Mex Army troops mixing it up with various miscreants. However my point is still valid, any full-auto M-16s that show up in Mexico most assuredly did not come from a "gun show" on our side of the border. I find it hard to believe that anyone with the patience to follow the Class III purchasing procedure to acquire a full auto firearm would turn around and pedal it to an undocumented third party. If for no other reason then fear of the ATF crawling up his "nether regions" with a microscope.
PS The same comment goes for RPGs, hand grenades, and mortars, anything like that found at a gun show would have been rendered thoroughly inert.