BREATH CONTROL
Breath control is important with respect to the aiming process. If the sniper breathes while trying to aim, the rise and fall of his chest causes the rifle to move. He must, therefore, accomplish sight alignment during breathing. To do this, he first inhales then exhales normally and stops at the moment of natural respiratory pause.
a. A respiratory cycle lasts 4 to 5 seconds. Inhalation and exhalation require only about 2 seconds. Thus, between each respiratory cycle there is a pause of 2 to 3 seconds. This pause can be extended to 10 seconds without any special effort or unpleasant sensations. The sniper should shoot during this pause when his breathing muscles relax. This avoids strain on his diaphragm.
b. A sniper should assume his firing position and breathe naturally until his hold begins to settle. Many snipers then take a slightly deeper breath, exhale, and pause, expecting to fire the shot during the pause. If the hold does not settle enough to allow the shot to be fired, the sniper resumes normal breathing and repeats the process. c. The respiratory pause should never feel unnatural. If it is too long, the body suffers from oxygen deficiency and sends out signals to resume breathing. These signals produce involuntary movements in the diaphragm and interfere with the sniper's ability to concentrate. About 8 to 10 seconds is the maximum safe period for the respiratory pause. During multiple, rapid engagements, the breathing cycle should be forced through a rapid, shallow cycle between shots instead of trying to hold the breath or breathing.
Firing should be accomplished at the forced respiratory pause.

The United States Army Marksmanship Unit Training Manual

SECTION ONE: FUNDAMENTALS OF PISTOL MARKSMANSHIP

D. BREATH CONTROL.
The correct method of breathing is an essential part of the shooter's system of control. Most pistol shooters know less about the proper method of breath control than of any of the other fundamentals.
The object of proper breath control is to enable the pistol shooter to hold his breath with a comfortable feeling long enough to fire one shot slow fire; five shots in twenty seconds timed fire; or five shots in ten seconds rapid fire without loss of the ability to hold still or concentrate on sight alignment.
1. To be Effective, Breath Control Must Be Employed Systematically and Uniformly: The ability to concentrate and maintain rhythm is aided.
a. Promote a steady hold: It is generally known that one must not breathe during aiming. Breathing is accompanied by the rhythmical movement of the chest, abdomen, and the shoulders. This causes the pistol to move about excessively, making it almost impossible to produce an accurate shot. Therefore, one must not simultaneously breathe and try to fire a shot, but must endeavor to hold the breath for a short period of time.
b. The physiological processes involved in breathing: The shooter however, must not view the breathing process solely from the movement of the chest and the gun. He must not forget that the process of breathing, which consists of a combination of processes which occur constantly in the human body, determine in general the condition of the human being. Therefore, proper breathing is of great importance during shooting exercises which last several hours. Incorrect breathing technique has an adverse effect upon shooting, especially if the concentration is disturbed by sensing of the need to breathe.
(1) During the process of breathing, there is an alternating increase and decrease in the volume of the chest, as a result the person inhales and exhales. A person inhales when the dimensions of the chest increase. Once inside the lungs, the air provides oxygen to the blood and in turn it absorbs carbon dioxide and aqueous vapors. Exhalation occurs when all the muscles relax, the diaphragm presses upward, and, under the action of the weight of the chest and the elasticity of the lungs, air is forced out of the body. Exhaling does not require muscular effort; it occurs as the result of the resiliency of the ribs and the muscular tissues and the elasticity of the lungs.
(2) When breathing calmly a person produces an average of 12 - 13 respiratory cycles a minute. Consequently, one respiratory cycle lasts 4 - 5 seconds. If one traces the respiratory cycle, it is not difficult to note that the strained position of inhalation is replaced very quickly by exhalation. The very next inhalation begins after a respiratory pause of 2 to 3 seconds, (figure 1-7) during which time the carbon dioxide accumulates in the lungs. The duration off the respiratory pause is determined by the ratio of oxygen and carbon dioxide in the air remaining in the lungs.

(3) The respiratory pause and the problems of the ventilation of the lungs are of great importance to the shooter. It is obvious that during aiming and applying pressure on the trigger, the breath must be held only after the shooter has exhaled, timing it so that the breath is held at the moment of the natural respiratory pause. During that time the muscles are not strained and are in a relaxed state.
c. A person can prolong by several seconds this respiratory pause, that is, hold his breath comfortably for 15 - 20 seconds, without any special labor and without experiencing unpleasant sensations. This time is more than adequate to produce a shot or shots. Experienced shooters usually take a deep breath before firing and then, exhaling slowly, hold their breath gradually, relax and concentrate their entire attention upon sight alignment and the smooth application of pressure on the trigger (Figure 1-8).

2. Recommended method
a. Prior to fire commands:
(1) When expelling the air from the lungs before aiming, no effort whatever must be exerted. The exhaling must be natural and free, as in ordinary breathing. The air must not be held in the lungs; incomplete exhaling before aiming leads to straining and to stimulation of the nerve centers regulating the breathing, and the shooters concentration on aiming is distracted.
(2) In order to make sure that during prolonged firing the interruption of the rhythm of breathing does not have an influence upon the shooter, the breath must not be held for an excessive period when trying to fire a slow fire shot. If the shooter does not produce a shot in 8 - 10 seconds, he must stop aiming and take another breath.
(3) Before holding his breath for the next shot he must empty his lungs well, taking several deep breaths. The same should be done between shots and strings of shots throughout the firing. This facilitates the lengthening of the respiratory pause before aiming and provides for regular rest between shots and strings. The oxygen level in the blood is slightly increased. As a result the shooter is relaxed and comfortable during all shooting without excessive and premature fatigue.
b. During the fire commands: Take a deeper than normal breath at the command, "READY ON THE RIGHT", take another at "READY ON THE LEFT", extend your pistol and take the final breath and exhale to the point of comfort at "READY ON THE FIRING LINE".
As the shooter gains experience in proper breath control, he will find that he will hold his breath, or extend his normal respiratory pause, without being too conscious of the action and allow intense concentration on sight alignment and trigger pressure. c. During actual firing: The shooter should not be conscious of the need to breathe. If during practice a shooter finds that he cannot hold his breath the twenty seconds necessary to fIre a timed fire string, he should make a practice of firing his timed fire strings in less than twenty seconds. However, if during a timed or rapid fire string, the shooter feels compelled to breathe, he should take a short breath quickly and continue to fire. This causes a lapse of concentration on sight alignment and should not be the normal technique used.