The reported discovery of a domestic arms laboratory in Mexico is a first and evokes many unanswered questions. The most important of these is why these arms trafficking rings would build weapons within Mexico at all, especially given the cost considerations of building them locally and the heavy and still largely unchecked flow of arms from the United States to Mexico.I think that is easy to answer. First, the "unchecked flow of arms" from the U.S. is a myth. The majority of American made arms that the cartels procured were military weapons provided to Honduras and El Salvador in the 1980's, and I would guess that the supply is drying up.
Second, and related to the first point, is that the cartels want automatic weapons, not civilian weapons.