## Figure M zeroed at meters M rezeroed at meters

(b) If an M16A2/A4 rifle has been zeroed at 25 meters using M855 ammunition and M193 ammunition is fired without re-zeroing, then the difference in the impact of the round on target varies between ammunition as shown in Table 5-6 and Figure 5-36.

 Sight Height Theta Bullet Drop Bullet Location M193 M855 M193 M855 M193 M855 0 2.568 0.00235575 0.00235575 0 0 -2.57 -2.57 25 2.568 0.00235575 0.00235575 -0.125 -0.14 -0.37 -0.39 50 2.568 0.00235575 0.00235575 -0.52 -0.58 1.55 1.49 75 2.568 0.00235575 0.00235575 -1.21 -1.33 3.18 3.06 100 2.568 0.00235575 0.00235575 -2.1 -2.4 4.16 4.31 125 2.568 0.00235575 0.00235575 -3.18 -3.82 5.85 5.21 150 2.568 0.00235575 0.00235575 -4.91 -5.61 6.43 5.73 175 2.568 0.00235575 0.00235575 -6.89 -7.78 6.77 5.88 200 2.568 0.00235575 0.00235575 -9.16 -10.4 6.82 5.58 225 2.568 0.00235575 0.00235575 -11.8 -13.4 6.50 4.90 250 2.568 0.00235575 0.00235575 -15.4 -16.9 5.22 3.72 275 2.568 0.00235575 0.00235575 -19.2 -20.8 3.74 2.14 300 2.568 0.00235575 0.00235575 -23.44 -25.3 1.82 -0.04 325 2.568 0.00235575 0.00235575 -28.3 -30.4 -0.73 -2.83 350 2.568 0.00235575 0.00235575 -33.8 -36.0 -3.91 -6.11 375 2.568 0.00235575 0.00235575 -40.13 -42.3 -7.92 -10.09 400 2.568 0.00235575 0.00235575 -46.78 -49.3 -12.25 -14.77

Table 5-6. M855 zeroed at 25 meters; M193 fired using M855 zero.

Table 5-6. M855 zeroed at 25 meters; M193 fired using M855 zero.

-■-Bullet Location M855 —Bullet Location M193

Figure 5-36. M855 zeroed at 25 meters; M193 fired using M855 zero.

NOTE: Both the M193 and M855 ball ammunition can be used in training and accurately function in the M16A2/3/4 and M4 carbine. Do not switch between the types during firing. Do not zero with one type, and then fire the other for any type of training. Due to the different characteristics of each round, zero with the same type ammunition used for training.

(7) A simple rule of thumb that will preclude any problem is to use only the ammunition specifically designed for each rifle (M193 ball ammunition for the M16A1; M855 ball ammunition for the M16A2/3/4).

b. External Ballistics. Soldiers must understand the basics of external ballistics so they can make necessary scope adjustments or hold compensations to allow them to hit the target. The external ballistic factors that affect bullet trajectory are:

(1) Gravity. The force of gravity on a bullet is constant regardless of its weight, shape, or velocity. The longer a bullet is in the air or the greater its angle from the vertical, the more effect gravity will have on its trajectory. (See paragraph 5-4 for more information on the effects of gravity.)

(2) Muzzle Velocity. Muzzle velocity is the speed of a bullet as it leaves the barrel, measured in feet per second. Muzzle velocity diminishes as the bullet gets farther away. The bullet reaches its maximum velocity 76 feet from the end of the rifle and slows down from there until it reaches the target.

(3) Air Resistance or Drag. Air resistance or drag immediately produces a slowing effect on a bullet.

(4) Altitude and or Air Density. The greater the altitude, the thinner the air and the longer the bullet will travel (with a correspondingly flatter trajectory). Each 5,000-foot elevation will raise the strike of the bullet 1/2 to 1 minute of angle.

(5) Temperature. Deviation from standard daytime temperature (59 degrees Fahrenheit/15 degrees Celsius) affects bullet trajectory.

(a) Cold air is denser than warm air meaning the bullet must travel through more tightly packed air particles. This causes the bullet to lose velocity resulting in the impact being lower than the intended point of impact. Cooler air also causes lower chamber pressure, which reduces the initial velocity.

(b) Warm or hot temperatures cause the strike of the round to move up.

(6) Trajectory. When a projectile exits the muzzle of a rifle, it drops from the line of departure, otherwise known as the center-bore line. As the projectile travels downrange, the velocity is decreased by air drag, giving way to the inevitable force of gravity. This effect creates trajectory.

(a) Line of Sight. The line of sight is an imaginary straight line extending from the shooter's eye through the telescopic sight, or rear and front sight, to the target.

(b) Line of Departure. The line of departure is an imaginary straight line extending from the center of the barrel to infinity.

(c) Zero Range. Zero range is where the projectile intersects the line of sight. It occurs at two points—one on the way up and one on the way down.

(d) Apex. Otherwise known as midrange trajectory, the apex is the point where the projectile is at its highest in relation to the line of sight.

(e) Bullet Path. The bullet path is the relationship of a projectile and the line of sight at any given range (normally expressed in inches).

(7) Wind. Although gravity and air drag are the only forces that act on the trajectory, other external factors influence the trajectory relative to the point of aim such as wind, altitude, temperature, humidity, and barometric pressure. Wind is by far the most significant.

(a) Because the bullet is moving through the air, the air moves the bullet. Wind deflection is always in the same direction the wind is moving. A wind blowing from the left will move the bullet to the right. Deflection decreases as the angle of the wind to the line of flight decreases. Reading and correcting for wind effectively takes practice, especially at longer ranges where accuracy in correcting is more critical.

(b) To shoot accurately in the wind, a shooter must know the wind velocity, wind direction, and the value of deflection at the range at which he is shooting. (See paragraph 5-4 for more information on wind direction.)

(8) Angles. Firing uphill or downhill normally causes the bullet to hit high relative to a horizontal trajectory. If the shooter is firing on an angle up or down at a slanted range of 100 yards, the point of impact will be higher than it would be for a level shot of 100 yards. How high depends on the angle.

(a) Gravity acts on a bullet only during the horizontal component of its flight (the distance from the shooter to the target measured as if they were both at the same level). Since the horizontal component will always be less than the slanted range, gravity will not pull the bullet down as far as it would if the range were level.

(b) The complicating factor in shooting uphill or downhill is that the wind will affect the shot over the entire slant range. The shooter should aim at the target as if it were 25 yards away and correct for wind as if it were 400 yards away. The correct method for shooting uphill or downhill is to adjust elevation based on the horizontal range, and correct for wind deflection based on the slanted range.

c. Terminal Ballistics. Bullet penetration depends on the range, velocity, bullet characteristics, and target material. Greater penetration does not always occur at close range with certain materials since the high velocity of the 5.56-mm bullet causes it to disintegrate soon after impact.

d. Bullet Dispersion at Range. Instructors-trainers must have a working knowledge of the effects of bullet dispersion and accuracy at various ranges.

(1) Minute of Angle. A minute of angle (a term used to discuss shot dispersion) is the standard unit of measurement used in adjusting rifle sights and other ballistic-related measurements. It is also used to indicate the accuracy of a rifle. A circle is divided into 360 degrees. Each degree is further divided into 60 minutes; therefore, a circle contains 21,600 minutes. A minute of angle is an angle beginning at the muzzle that would cover 2.54 centimeters at a distance of 91.4 meters (Figure 5-37). When the range is increased to 182.8 meters, the angle covers twice the distance, or 5.08 centimeters. The rule applies as range increases—7.62 centimeters at 274.2 meters, 10.16 centimeters at 365.6 meters, and so on.

Figure 5-37. Minute of angle.

(2) Increase of Shot-Group Size. Just as the distance covered by a minute of angle increases each time the range increases, a shot group can be expected to do the same. If there are 2.54 centimeters between bullets on a 25-meter target, there will be an additional 2.54 centimeters of dispersion for each additional 25 meters of range. A 2.54-centimeter group at 25 meters (about 3.5 minute of angle) is equal to a 25.4-centimeter shot group at 250 meters (Figure 5-38).

10-INCH SHOT GROUP

10-INCH SHOT GROUP

0 2&M 50M 75M 100M 12SM I50M 1 75M 200M 225M 2S0M

Figure 5-38. Increase in shot-group size as range increases.

0 2&M 50M 75M 100M 12SM I50M 1 75M 200M 225M 2S0M

Figure 5-38. Increase in shot-group size as range increases.