Well that's how it works with civilian controlled guns in a weird way. All it takes is a few thousand US civilian firearms, and the US gets listed as the main supplier of Mexican cartels.

We'll just ignore that it's a few thousand out of millions, and doesn't account for the military grade gear most cartels get. But whatever, it's a convenient talking point.

Seriously though, the "fact" that the US supplies most of their guns through smuggling is based off a few thousand guns given to the ATF in Mexico City. A few thousand guns, that Mexico got to go through first, and most of the ->traceable<- guns were from the US. These are semi-auto guns, while the Cartel militias use full auto military gear.