Snipers don't hold their breath. The first thing to go when you hold your breath is vision. There's two natural respiratory cycles where you're the most still, at the top of your breath and at the very bottom, it's better to break the shoot (with a precision rifle) at the bottom of the breathing cycle because your lungs act like balloons when they're full of air you have extra movement rather then if they were deflated at the bottom. Also when you hold your breath your heart rate goes up because it has to work faster to do the same job. Breathing is the most normal thing in the world you do it all day everyday, while you sleep while you eat. I don't know why people think just because you have a gun in your hand people automatically think they should stop breathing.