Mexican drug gangs looking for weapons powerful enough to stop a vehicle, penetrate a bullet-resistant vest or confront an army detachment need look no further than the Houston area's 1,500 gun shops, where merchandise is priced to move.

Guns like the Barrett M-82 sniper rifle, the AK-47 and Bushmaster .223 are among those favoured by cartel hitmen that slaughtered some 6,300 people in Mexico border cities like Ciudad Juarez and Tijuana last year.

In Mexico, gun sales are regulated by the government and require citizens to apply for permits, which are rare and expensive. But deadly weapons are in ready supply at a substantial discount for those Mexicans willing to drive 563 kilometre from the border. "They can do the math," said Dewey Webb, Special Agent in Charge of the Houston field division of the US Bureau of Alcohol, Tobacco, Firearms and Explosives. "They can come a few hundred miles north and get a lot better price on it." For example, the Belgian-made FN 5.7-caliber pistol is known to gangs as a "cop killer" because it can fire a round through a Kevlar bullet-resistant vest. It goes for €571 in the Houston area, compared to €1,000 in a border store, said Mr Webb.

The Mexico border violence is starting to spill into the US. In Tucson, Arizona, a two hour's drive from the border, residents have seen a rash of home break-ins and assaults from gangs linked to the lucrative drug trade.

So far, the violence has not disrupted businesses in border towns like El Paso or Ciudad Juarez, said Bob Cook, president of the El Paso Regional Economic Development Corp.

With over 235,000 manufacturing jobs and 70 Fortune 500 companies, the two cities boast one of the biggest manufacturing hubs in North America, including auto part manufacturers like Bosch and medical device makers like Cardinal Health.

"It's not the dollar outweighing the security issue," Cook said. Even though the murder rate in Ciudad Juarez has skyrocketed, owners of the 350 plants there "are coming to the conclusion that criminal activity does not directly target them," he said.

US authorities say Mexican drug gangs, like Los Zetas, who act as enforcers for the Gulf Cartel, now possess a sophisticated, high-powered arsenal that gives them the firepower to take on the Mexican army.

There's also the AK-47 assault rifle, known by drug gangs as the "cuerno de chivo" or goat's horn because of its distinctive curved bullet magazine. Drug gangs like the compact gun because a dozen of them will fit into the trunk of a car.

And brand-conscious buyers ask for the Bushmaster .223-caliber semi-automatic rifle by name because it has a coiled snake stamped on it.

Guns bought in Houston through 'straw purchases' have been traced to dozens of murders in Mexico.

One or two are purchased at a time, making them harder to track because of the sheer number of seemingly legitimate buyers who only buy a few guns from any given store. One case that was tracked involved an individual named John Phillip Hernandez, who on July 12, 2006, walked into the Academy firearms dealer in Houston and bought a Beretta rifle and two Bushmaster rifles, according to case documents.

Seven months later, one of the guns was used in an attack on government offices in Acapulco, Mexico, in which four police officers and three secretaries were killed.

To Mexican gangs, it's a logical flow. They carry tons of cocaine, marijuana and other narcotics, and also smuggle immigrants, from Mexico to Houston, a major US drug distribution hub. On the return trip they cart back guns and ammunition, added Mr Webb.