Instructor Graham Dunne is holding up some printouts with faces on them. He tells his students they're smaller than real heads.
"Here's some useless knowledge from being a sniper," he says. "The average human head is 6 inches across by 10 inches high. These are probably half that."
We're at the Flatrock Regional Training Center in Commerce City, Colorado. Usually the people training here are law enforcement, but today they're teachers, principals, bus drivers, coaches and school administrators — 13 of them.