KABUL, (AP) - The second shooting of Western troops by one of their Afghan counterparts this month has highlighted the potential hazards of a push to speedily expand Afghanistan's army and police forces in the next few years. On Tuesday, an Afghan army sergeant opened fire at an army base in northern Afghanistan, killing two American civilian trainers before being shot dead. That followed an attack in the south on July 13, when a soldier killed three British troopers, including the company commander, with gunfire and a rocket-propelled grenade in the dead of night.