Look in the mirror

Guns from the US, sure. But you also need to look at those that are the result of Mexico itself in addition to you neighbors to the South.

Smuggling of firearms into Mexico
From Wikipedia, the free encyclopedia

Mexicans have a right to own firearms,[1] but legal purchase from the single Mexican gun shop in Mexico City, controlled by the Army, is extremely difficult.[2] Guns smuggled into Mexico are sometimes obtained at gunshops in the United States and carried across the US-Mexico border.[3][4] In other cases the guns are obtained through Guatemalan borders[5] or stolen from the police or military.[6] Consequently, black market firearms are widely available. Many firearms are acquired in the U.S. by women with no criminal history, who transfer their purchases to smugglers through relatives, boyfriends and acquaintances who then smuggle them to Mexico a few at a time.[7] The most common smuggled firearms include AR-15 and AK-47 type rifles, and FN 5.7 caliber semi-automatic pistols. Many firearms are purchased in the United States in a semi-automatic configuration before being converted to fire as select fire machine guns.[8] Mexico seized in 2009 a combined total of more than 4,400 firearms of the AK-47 and AR-15 type, and 30% of AK-47 type rifles seized have been modified to select fire weapons, effectively creating assault rifles.[9]
There are multiple reports of grenade launchers being used against security forces,[10][11][12][13][14][15] and at least twelve M4 Carbines with M203 grenade launchers have been confiscated.[16] It was believed that some of these high powered weapons and related accessories may have been stolen from U.S. military bases.[17][18] However, most U.S. military grade weapons such as grenades and light anti-tank rockets are acquired by the cartels through the huge supply of arms left over from the wars in Central America and Asia. It has been reported that there have been 150,000 desertions from the Mexican army during 2003 to 2009. Stated another way, about one-eighth of the Mexican army deserts annually.[19] Many of these deserters take their government-issued automatic rifles with them while leaving. Some of those weapons originate from the USA.[20] It has been determined that at least some of the M203 grenade launchers and M16A2 assault rifles cited above are of counterfeit origin manufactured for the cartels, possibly to resemble the weapons carried by the Mexican Special Forces.[21]