Perhaps I'm not thinking creatively enough about this, but I don't see how the government can do much about this without outlawing the guns that drug runners in Mexico want, which is not a likely possibility to put it mildly. Each gang has 30-40 people buying guns now, which keeps them more than amply armed. The best way to crack down on them would seem to be to start keeping a closer eye on people who make a lot of gun purchases, but that even that could cause enough of a political storm to be unfeasible. (Here's where I expose my ignorance on the subject, but I assume that if I were to make as many legal gun purchases as is possible in a given time frame, as long as I don't brag about selling them to Mexican gangs or start shooting the guns in public, no one will start investigating me.) But even if the US starts focusing on people who buy suspiciously large amounts of guns, couldn't the gangs just expand their ranks of ants five- or ten-fold, so as to make the frequency of the illegal purchases indistinguishable from your typical gun buyer? And if the straw buyers on retainer are already making so few purchases as to be not markedly different from the law-abiding gun fans, then where do we even begin?
So all this leads me to suspect that while the US should be doing more to crack down on the arms trade from a moral standpoint, within the present political boundaries, it's not realistic for US gun control policies to be a driver of a safer Mexico. Bummer.