Interesting comment on the black market in military equipment. I wonder how much stolen military equipment,especially weapons and ammo, have been sold to the drug cartels. Over the years a good amount of miltary equipment has been stolen, particularily in the early 70s. You may remember the scandal in the late 80s over the extemely lax controls the 2d Ranger Bn had over its very generous ammunition allocation. One of the bn's supply sergeants was caught selling ammunition and grenades to biker gangs.
Human nature being what it is, people will give in to temptation especially if the money is good. No doubt some of the Mexican drug cartels weapons come from the US, if only because it is convenient to spend some of the proceeds of drug sales in the US on gun purchases in the US. Undoubtedly the cartel also buys weapons on the international black market. As you mentioned; all the shouting by various partisans in America obscures or does not address the real problem which is stopping the flow of whatever wepons are going into Mexico from the US and dealing with the insatiable demand for drugs in America. "Sealing" the border is a component of this but that is a comment for another time.