BAGHDAD, Iraq — Army Spc. Christiansen Cory was talking with another soldier when he felt something smack his forearm. “We were standing around and heard a pssssst,” he said. “It felt just like a big rock had been thrown at me.” But when Cory looked down, he discovered it wasn’t a rock that hit him. At his feet was a bullet from a Kalashnikov rifle. It was still warm. The round likely came from an Iraqi who fired it into the air in celebration of a wedding, the birth of a child, or, for nothing in particular. Every day, sometimes...