Between Jan 2000-Dec 2006: More than 163,000 military members were criminally processed during former president Vicente Fox’s 6 years term of office. The majority of the crimes were: [the list includes abuse of power, homicide, embezzlement, kidnapping, bank robbery, illegal possession of firearms and health crimes [essentially organized crime].Another slide in that same DEA PowerPoint presentation states that the Mexican military reported an average of 1,200 desertions per month in 2006.
I would also say that neither article I linked, nor the one heading this thread, attribute the whole problem to legally transferred arms from the U.S. Nor did I intend to infer that.
Are you saying that the Mexican Army uses US military small arms? Aren't all of those serialized? If in fact the weapons the gangs use do not (for the most part) have serial numbers, then the scenario you describe, while I am sure it happens, does not follow in most cases, unless of course they were removed. I just don't see why anyone in Mexico would bother removing them. It's easier just to buy a dispensation from the Federales.