How do we know

they are going to the cartels?

With all the violence down there, I would imagine that there are Mexican citizens that have said "the heck with the law, I'm going to protect my family"

Since the cartels can very likely obtain "Full Auto" AKs out of South America at a lower cost with less hassle.

Perhaps the businesses, and people of Mexico have grown weary of the government's ineffectiveness.

Think about it for a moment, when Barky got elected, and people thought he would push gun control, sales nearly doubled here, Ammunition sales sky rocketed. Down in old Mexico otherwise law abiding people are afraid, and are going to find ways to protect themselves

Its human nature