Captured Taliban ammunition and automatic weapons appear to be from stocks supplied by the United States to Afghan government forces. Well, surprise, surprise. From the New York Times:
Arms and ordnance collected from dead insurgents hint at one possible reason: Of 30 rifle magazines recently taken from insurgents’ corpses, at least 17 contained cartridges, or rounds, identical to ammunition the United States had provided to Afghan government forces, according to an examination of ammunition markings by The New York Times and interviews with American officers and arms dealers.
The presence of this ammunition among the dead in the Korangal Valley, an area of often fierce fighting near Afghanistan’s border with Pakistan, strongly suggests that munitions procured by the Pentagon have leaked from Afghan forces for use against American troops.
With a government thoroughly infiltrated by warlords whose very history is of playing both sides of the street, is this any surprise? The insurgency is no longer the exclusive preserve of the Taliban. It's long morphed into a coalition of groups - Islamist, nationalist, criminal or those simply aggrieved and seeking revenge.
The United States has been criticized, as recently as February by the federal Government Accountability Office, for failing to account for thousands of rifles issued to Afghan security forces. Some of these weapons have been documented in insurgents’ hands, including weapons in a battle last year in which nine Americans died.