You're just lucky Game Mechanics favor the player. Were they not, the Rangers would just AMR you in the head from beyond perception range.
Actually, it's easier than you'd think. Very simple to have a sniper that insta-kills someone, or at least deals a lot of accurate damage without being detected.
Point Lookout, for one, had a sniper that from a distance he didn't even appear in compass, would shoot rapid fire with sniper rifle, always scoring headshots to the player. This was after escaping the Chinese-execution-chamber-thing. A real nasty one that guy, especially with difficulty-increasing mods.