Gun smuggling to Mexico
The Arizona Republic carries a story on it.
I wouldn't doubt that there's some of that, but I'm told by Border Patrol friends that the smugglers prefer full auto AKs, which you don't find in the typical American gun store, nor get without a government clearance if you do. And the story goes WAY overboard with scare lines like:
"Cartel operatives flood Arizona to buy semi-automatic assault rifles, grenades, plastic explosives and rocket launchers in bulk. "
Hmmm.... I dunno about anyone else here, but I can't think of a place that sells grenades, plastic explosive, or rocket launchers, let alone in bulk. If they are getting any, there's only one place they could come from: government supplies.