The government of Mexico is complaining that drug cartels (which for all practical purposes own the government of Mexico) are buying guns in the US.
"Although firearms can be bought on the black market in Mexico, it is very difficult to purchase them legally." Natch. The accustomed mordida, tip, for not having your car inspected was, last time I went across, $10. Slip that to the border inspector and it doesn't matter what is in the trunk of your car -- a dozen guns, a few bodies, a nuke -- you're in without inspection.
[Update: it was in the early 90s]