That .50 cal anti-aircraft weapon

I just came across an article here, that breaks down some of the curious details surrounding the seizure of a anti-aircraft machine gun found mounted in the bed of a pick up truck.

Here's the more relevant section:

One of the most worrisome weapons yet was seized this week just south of Nogales, Ariz.: a powerful gun mounted on the back of an SUV and protected by a thick metal shield. Police said it belonged to one of the Beltran Leyva drug gangs.

Mexican and U.S. authorities disagree on just what type of gun it was. Federal police coordinator Gen. Rodolfo Cruz maintains it was .50-caliber anti-air craft machine gun. ATF, the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, said it was an unmodified .50-caliber semiautomatic rifle made by TNW, a U.S. firearms manufacturer.

ATF investigators traced the gun - along with seven others seized at a house in Sonora state on Monday - to suppliers in the United States, said Bill Newell, special agent in charge of the ATF in Arizona and New Mexico.

While crudely built, the truck-mounted rifle would give traffickers a powerful advantage against lightly armed police, Newell said: A gunman could protect a whole convoy with sweeping fire while protected by the metal shield.

"Imagine being a two- or three-man police team at a rural checkpoint and these guys roll up with this thing," Newell said. "You'd be slightly intimidated, wouldn't you?"