Automatic weapons sold legally in the US are flowing over the border into Mexico to arm narco terrorists who control the drug supply that floods back.

Deadly drug cartels are using 50-calibre rifles - used to rip through armoured vehicles - against police and the military along with AK-47s and AR-15s in gang wars that are claiming tens of thousands of lives.

Journalist Ioan Grillo, who has reported for more than 20 years from Mexico on cartels and narco wars, says an iron river flowing from the US is arming violent drug gangs wreaking death and destruction.

In an interview on this week's Crime World podcast, Grillo says that a third of Mexico's two million square kilometres and population of 130 million people are living under serious narco cartel problems. Another third has a cartel presence, while other parts of the country are only as violent as Belgium. 

But it is the level of firepower in the hands of the narcos that causes the real casualties.

Crime World, with Nicola Tallant, is a weekly Iri…