What’s an example of a “crucial detail” from your line of work that outsiders might notice but wouldn’t consider significant unless an insider pointed out how/why it was crucial?
Not my "line of work," but I've been trained.
When bullets strike a smooth, hard surface, they don't "reflect" the way you learn in high school physics. They tend to skip like a stone on the surface of a lake. This has 2 vital consequences.
/1
Clarification: this applies to bullets that strike the surface at a shallow angle.
First, avoid positioning yourself within 2 feet of a wall.
Second, it can be feasible, esp. with practice, to hit targets not in your line of sight by skipping bullets off an adjacent surface.
/2
If an ill-trained criminal/terrorist is firing at you while leaning against a wall, you can stay behind cover and skip bullets along the wall in front of him.
/3
Another surprising detail from the gun world is a discovery that goes back to the 1500s and can be confirmed with simple geometry.
When you fire at a target uphill *or* downhill from you, you aim low, that is, you aim below the target.
/5