"When the bullet pierces the painted steel," Garreth said, "it bends the steel inward. The paint isn’t flexible, so it shatters. Some of it vaporizes. Result is bright, shiny steel, visible around the hole. The hole itself is no bigger than the tip of your finger. It’s the spalling that visually identifies a bullet hole, so we have to cover it. And we want as tight a seal as possible, because we don’t want to be setting off sensors."