Have any of ya'll read Gen. McCaffrey (by my estimation no rabid gun-banner)'s AAR from his Mexico visit, summing up the current narco-insugency in Mexico?:
Scrol to page 6, Section 6-D
http://www.mccaffreyassociates.com/pdfs/Mexico_AAR_-_December_2008.pdf
(he does claim these are "automatic weapons" smuggled from the US but I'm willing to believe that's conflated with "semi-automatic"; as many have argued here, losing full-auto doesn't make much of a difference in lethality.) He claims that 90% of these weapons are smuggled across the US border, most purchased from licensed dealers, and that US law enforcement is unresponsive to concerns about weapons, bulk cash, and precursor chemicals crossing the border.
One such dealer is being prosecuted for selling as many as 600 to smugglers.
http://www.nytimes.com/2009/02/26/us/26borders.html?_r=1