How many guns were not submitted to the ATF because the Mexican government knew they were either locally-sourced or from South America?
The US govt gave and sold a boatload of weapons to the Mexican Federales over at least a 20 year period to bolster the Mexican military. They have a high rate of desertion and when they go over the fence the soldiers take every piece of gear they have. A good number of these folks have joined the drug cartels and they are using these weapons. Of course the BATFE isn’t going to tell you that.