I've had a few instance where my weapons become ineffective or the enemies become bullet proof. It doesn't matter which weapon I use when it happens.

The first time it happened I shot an enemy with my sniper rifle which only served to get his attention. He ran behind a building so I run up to him with my pistil and shot him in the face several times with no effect. I then pulled out my assault rifle and blasted away, still no effect. Finally one of my AI team shot him and killed him.

Has anyone else had this problem?