Private Barnes has a right shoulder sleeve insignia (combat patch) on his right shoulder, indicating that he has already completed a combat deployment. It is highly unlikely that he would still be a Private on his second deployment. Additionally, a Private would never be in charge of a security escort detail.
When Richie (Bill Murray) and Bombay Brian (Bruce Willis) go to the desert to do deliver the weapons/ammo and collect the money, one of the tribal elders tells them that they are being forced to grow poppy. However, Paktya province is virtually poppy-free, as the altitude is too high for that crop. On the other hand, it is common to find huge fields of marijuana plants.