KEY POINTS:
The agent raised the machine gun to his shoulder and let rip a deafening hail of shots that smacked through a bullet-proof vest, punched holes in a car door and spat up a plume of sand in the levee behind.
Taking the plugs out of my ringing ears, I walked over the gritty desert firing range to look at the damage from the Kalashnikov.
The traffic of guns of this type from the United States to the warring Mexican drug cartels is the less-reported flip side of the drugs trade north from Mexico and South America.
I have covered crime on the US-Mexico border for several years, examining the debris from the firing and explaining the effects.
Last year, Mexican cartels murdered more than 2500 people in an all-out war for lucrative drug routes to the US worth billions of dollars.
Many were shot using assault rifles readily and legally available in the US, and trafficked to Mexico, where gun sales are banned.
Earlier, I got into my pickup truck to follow the route of one semi-automatic Kalashnikov bought at a Phoenix gun store.
It turned up a year later in a raging gun battle around the mining town of Cananea in northern Mexico that killed 23 people.
The rifle, like thousands of others, was bought in what they call a "straw purchase" - where a third party is paid to buy guns to order for a cartel "gatekeeper" - then driven over the desert to Mexico.
I crossed through the tiny border town of Naco, Arizona, where in 1916 troops under General John Pershing had pursued bandit-turned-revolutionary Pancho Villa into Mexico during the Mexican revolution.
A road sign near the adobe border garret read "Warning: Firearms and Ammunition Illegal in Mexico".
I saw a Mexican immigration agent rearranging a few traffic cones on the road, but no one stopped me to ask for my passport, much less check the truck.
I could have had a case of rifles in the back.
It took less than an hour over a narrow, winding road to reach Cananea, where on May 16, 2007, armed assailants drove into town in a convoy. Once they had methodically set up roadblocks, they started the killing, shooting dead five police officers and two civilians.
The assailants kidnapped ranch hands and commandeered horses, which they used to escape into the western Sierra Madre mountains. Police and troops were in hot pursuit and killed 16 of them before the day was out.
ATF agents work closely with Mexican authorities to stem the flow of weapons south under Project Gunrunner, using a software programme called e-trace to track guns from Mexican crime scenes to the dealers who sold them in the United States.
The ATF frequently seizes military-grade weapons headed for the hit men, including assault rifles and ammunition.
