AR-15 Handbook. Erik Lawrence

      Figure 2-2 Direct-Impingement Gas System during firing

      CHAPTER 3

      BALLISTICS

      BALLISTICS

      The study of the flight characteristics of projectiles (bullets).

      TRAJECTORY

      The path of a moving body (bullet) while in flight.

      HOLDS

      (HOLD-OVER/HOLD-UNDER AND WINDAGE HOLD LEFT OR RIGHT)

      How far you must place your aiming point over, under, left, or right of the desired point of impact to achieve a hit.

      EXTERNAL BALLISTICS

      Shooters 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.

      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 causing the bullet to lose velocity and 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 sights, 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 (maximum ordinate). 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.

      8.ANGLES

      Firing uphill or downhill at an angle of 45 degrees or more 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 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.

      EXAMPLE

      Shooter is 300m (984ft) up the side of a ridgeline and wants to engage a target at the base of that ridge with a horizontal distance of 50m. The target’s actual distance from the shooter is slightly over 305m. Shoot the point of aim as though the target is at 50m, using wind calls for a 300m shot. See Figure 3-1 for an example.