Why on Earth would an army provide its enemy with ammunition? So they would use it, of course. The United States wanted the North Vietnamese to use the ammo they provided because they would take out the weapon (and maybe even the person) using it.
There was no unconventional war like the one that played out behind the scenes of the greater war in Vietnam. One small aspect of that hidden war was Project Eldest Son, a plan that would take out the enemy's individual infantry rifles using its own ammunition.
It was carried out by a U.S. military entity called the Studies and Observations Group, the Special Forces unit that was behind many of the top secret missions and operations inside the Military Assistance Command Vietnam. The unit was in many of the major battles and offensives of the war, including the Tet Offensive and the Easter Offensive. But Project Eldest Son was different. It was a slow burn, a subtle influx of materiel into the enemy's supply and ammunition depots, with one marked difference – one that wouldn't show itself until it was too late.
Starting in 1967, the United States and the MACV-SOG began sending the Communist forces throughout the area ammunition for the AK-47, machine guns, and even mortars. They all looked ordinary, but they didn't work like any ordinary ammo – and they weren't just duds, either. These rounds were filled with high explosives, enough not just to fire the projectiles, but enough to destroy the weapon and severely wound the shooter. For the mortar rounds, the explosives together could kill an entire mortar crew.
After a while, the United States hoped the Vietnamese Communists would be afraid to use their own weapons and ammo. Killing the enemy was a good side effect, but the SOG needed some of them to survive.
For two years, special operators all over Vietnam would capture ammunition and supply centers, infuse cases of ammo with the faulty ammunition and then let it end up back in the hands of the enemy. Like seemingly everything in Vietnam, you never knew what might be booby-trapped. Eventually, the SOG would have to warn U.S. troops against using Communist weapons and ammo over the defective new M-16 to prevent the explosives from killing friendlies.
The program only ended because it was leaked to the media in the West, but even so, the efficacy of the program was never fully known.