MEXICO CITY — As recently as a year or two ago, commandos fighting for the Mexican drug cartels often would rather flee than confront security forces.
But an influx of combat weapons — purchased at U.S. gun shops and shows or stolen from Central American munitions stockpiles — and a vast supply of ammunition now enables them to fight, and sometimes outgun, army and federal police units.
Cartel squads toss hand grenades, fire rockets and spray security forces with high-caliber gunfire. They sometimes have 10 times the ammunition of federal forces.
The arsenals give them a greater ability to threaten the state. The resulting mayhem steadily ripples northward as civilian "narco refugees" flee areas of extreme violence.
A sample of the growing firepower of Mexico's seven major drug cartels is on display at the military warehouse on the outskirts of the capital, where seized assault rifles, machine guns, high-caliber weapons and anti-tank rockets are stored.
"As you can see," Gen. Antonio Erasto Monsivais said as he led a visitor around, "they have weapons capable of high destruction. They can confront the armed forces, whereas before they used to flee."
Monsivais cradled a menacing weapon with a bulbous chamber, a South African-made multiple grenade launcher that fires explosive rounds at rat-a-tat speed. The device empties its chamber as fast as the trigger is pulled.
"It's designed to level an area," the general said, "not to hit a specific target."
The warehouse holds plenty of kitsch weapons: pistols encrusted with rubies or assault rifles engraved with images of cobras, pumas and religious icons.
Security forces also have seized tens of thousands of venerable assault rifles such as the AK-47 and its American cousin, the AR-15. U.S. and Mexican experts say 90 percent of such semi-automatic rifles are smuggled from the United States.
The vast majority of U.S. states permit sales of semi-automatic assault rifles. Only the District of Columbia and a handful of states — notably California, New Jersey and Connecticut — ban or sharply restrict sales of such weapons. There's almost no restriction on sales of ammunition.
"They take off the stock and they cut them down. They can put them under the dashboard for quick access," said Walter M. McKay, the Canadian director of the Center for Professional Certification of Police, a Mexico City training organization.
More exotic weapons such as Barrett .50-caliber sniper rifles and a Belgian-made handgun known as the FN Five-seveN, whose high-velocity ammunition has brought it the moniker "cop killer" in Mexico, are finding a bigger place in criminal arsenals.
"The .50-calibers are of growing concern. The cartels are looking at them as an anti-personnel weapon. We've actually seen them mounted on the backs of pickup trucks," said William G. McMahon, the deputy assistant director of the Bureau of Alcohol, Tobacco, Firearms and Explosives in charge of the Southwest border.
Drug gangs deploy the high-caliber weapons from bridges to disable vehicles in military or police convoys, firing into the engine block of the lead vehicle, then attacking the fleeing occupants with grenades and smaller-caliber assault rifles.
The powerful .50-caliber rifle is a fearful addition to the criminal arsenal.
"It fires a very big round, as big as your hand. The bullet is half an inch across," said Tom Diaz, senior analyst at the Violence Policy Center in Washington, which advocates stricter gun laws. "If you just fire it out the window, the bullet would go about four miles."
"We know it's one of the top guns that are smuggled into Mexico."
McMahon said cartels were snapping up a certain kind of weapon from U.S. suppliers. "They are looking for the highest caliber, highest capacity weapon they can, because they are at war with the military," he said.
As cartels build up their weapons stocks, a bloody new phase unfolds in a drug war that's already left nearly 30,000 victims since late 2006.
A benchmark occurred July 15 when La Linea, a gang based in Ciudad Juarez, a border city racked by violence, detonated a Ford Focus. It was the first time cartels had used a sophisticated, remotely triggered car bomb made with plastic explosives.
A second car bomb detonated outside police headquarters in Ciudad Victoria, capital of northern Tamaulipas state, on Aug. 6, although no one was injured.
While the bombs contained limited plastic explosives, they had the sophistication of much larger devices, U.S. experts said. Bigger bombs could be in the offing.
"The line in the sand is drawn at the consideration of: 'Do we need to use this or is this something we hold in reserve for an extreme act?' " noted Samuel Logan, Latin America regional manager with iJET Intelligent Risk Systems, an Annapolis, Md., consultancy.
Far more common is the increasing use of hand grenades. Central America is awash in weapons from the civil wars of the 1980s and '90s, and several Mexican cartels have stockpiled grenades stolen from military depots in Guatemala and elsewhere in the region.
Grenade attacks occur several times a week in Mexico, sometimes aimed at television stations, newspaper offices or rival cartels.
Intelligence analysts say the stockpiles seem to be expanding.
"I'd be very surprised if there weren't tens of thousands of grenades from Central America" in Mexico, said a former senior CIA official who follows the drug war closely but couldn't be named because his current employer didn't authorize him to speak publicly.
Grenades sell for about $500 apiece on the Mexican black market. Some are homemade, using conventional hulls or jackets with blasting agents and explosives from the mining industry.
The ATF joined Mexican law enforcement in 2008 to create a Combined Explosives Investigative Team to help Mexico analyze blast scenes. Last year, the team investigated 114 grenade blasts, McMahon said, and found that 102 of them were of grenades manufactured before 1990.
Some Mexican and U.S. experts say mercenary cartel armies lack mastery of their increased firepower, but compensate by shooting more lead.
"These guys feel they have more possibility of survival if they jump from a vehicle and pfft! pfftt! pfft! They just spray gunfire," said Sigrid Arzt, a former senior national security adviser to President Felipe Calderon.
"Their tactic is a high volume of fire with little accuracy," Monsivais said.
Post-battle investigations show cartel gunmen routinely have 10 times the ammunition that police and soldiers are issued.
"Ammo is key for these cartels," McMahon added, noting that purchasers for the cartels can load up at U.S. gun shops. "You don't have to fill out any paperwork on ammunition. There's no background check on it."
In many U.S. states, a law-abiding resident can buy large quantities of .50-caliber sniper rifles and cheap foreign-made assault weapons. Unlike with handguns, U.S. gun dealers aren't required to report multiples sales of assault rifles to the ATF.
Drug cartels use networks of "straw buyers" — people without criminal records — to obtain weapons in U.S. gun shops, a point that's an irritant in U.S.-Mexico relations.
The cartel networks that move marijuana and narcotics into U.S. cities also are employed to stock arsenals back in Mexico.
"They just reverse the trade to get weapons and cash back south," McMahon said.
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