It would not make one bit of difference whatsoever if the border were closed completely and if every firearm were stopped from "going south," the Mexican narco gangsters would still get all they wanted. Foreign gangs would merely smuggle them into Mexico from foreign countries.
The Mexican narco gangsters have literally billions of U.S. dollars, cash money on the barrel head, sitting around in their "coffers." They would have no trouble at all, buying all the firearms, ordanance, and ammo they wanted.
There is a gigantic network of foreign smugglers, worldwide, dealing in the trade of anything anyone wants, so long as the money is there to pay for it.
Eric Holder and The Great Lord Obamacus Rex The Magnificent, The One Mighty And Strong, The Most Righteous Savior Of Mankind, are merely using the "stop guns to Mexico" cry as a lying ruse to eventually disarm American citizens.
Anyone interested, BTW, in reading an excellent book on the international smuggling trade should buy a copy of "McMAFIA, A Journey Through The Global Criminal World," by Misha Glenny, Alfred A. Knopf Publs., (c)2008.
It's an eye opener!!
L.W.