ORLANDO, Fla. (AP) — The shooter rapidly fires through the front doors of an elementary school with an assault rifle and blasts his way down the hallway. Screaming children are running for their lives or frozen in fear. Teachers quickly try to decide: barricade the doors, or make a run for it with their students?

Police officers arrive with guns drawn, working their way through the school. Finally they confront the shooter and end the threat.

Using cutting-edge video game technology and animation, the U.S. Army and Homeland Security Department have developed a computer-based simulator that can...