It looks to me like Our government must be selling arms to their government. The bad guys are probably getting them when the Mexican police tuck tail and run. When you see that many Barretts, that tells me that it isn't civilian ownership of Barrett's that is supporting the gangs in Mexico but rather our own, and their own, governments.
More reason why only civilians should be permitted to own guns. The government is just not responsible enough to be trusted with them.