For Walk through Wednesday, we are continuing our discussion on ballistics. Specifically we will be talking about external ballistics. The reference for this discussion is TC 3-22.9, change 1 dated January 2017.
External ballistics is the study of the projectile from the time it exits the barrel to the moment before it impacts the target, whatever it happens to be. TC 3-22.9 defines it as: “the study of the physical actions and effects of gravity, drag, and wind along the projectile’s flight to the target.”
The following terms and definitions are used to describe the actions or reactions of the projectile during flight. This terminology is standard when dealing with any weapon or weapon system, regardless of caliber.
Axis of the bore (Line of Bore) – the line passing through the center of the bore or barrel.
Line of sight (LOS) or gun target line (GTL) – a straight line between the sights or optics and the target. This is never the same as the axis of the bore. The LOS is what the Soldier sees through the sights and can be illustrated by drawing an imaginary line from the firer’s eye through the rear and front sights out to infinity. The LOS is synonymous with the GTL when viewing the relationship of the sights to a target.
Line of elevation (LE) – the angle represented from the ground to the axis of the bore.
Ballistic trajectory – the path of a projectile when influenced only by external forces, such as gravity and atmospheric friction.
Maximum ordinate – the maximum height the projectile will travel above the line of sight on its path to the point of impact.
Time of flight – the time taken for a specific projectile to reach a given distance after firing.
Jump– vertical jump in an upward and rearward direction caused by recoil. Typically, it is the angle, measured in mils, between the line of departure and the line of elevation.
Line of departure (LD) – the line the projectile is on at shot exit.
Muzzle – the end of the barrel.
Muzzle velocity or velocity – the velocity of the projectile measured at shot exit. Muzzle velocity decreases over time due to air resistance. For small arms ammunition, velocity (V) is represented in feet per second (f/s).
Twist rate – the rotation of the projectile within the barrel of a rifled weapon based on the distance to complete one revolution. The twist rate relates to the ability to gyroscopically spin-stabilize a projectile on rifled barrels, improving its aerodynamic stability and accuracy. This effect is similar to The twist rate of the M4 or M16-series weapon is a right hand, one revolution in every seven inches of barrel length (or R 1:7 inches).
Shot exit – the moment the projectile clears the muzzle of the barrel, where the bullet is not supported by the barrel.
Oscillation – the movement of the projectile in a circular pattern around its axis during flight.
Drift – the lateral movement of a projectile during its flight caused by its rotation or spin.
Yaw – a deviation from stable flight by oscillation. This can be caused by cross wind or destabilization when the projectile enters or exits a transonic stage.
Grain (gr) – a unit of measurement of either a bullet or a propellant charge. There are 7000 grains in a pound, or 437.5 grains per ounce.
Pressure – the force developed by the expanding gases generated by the combustion (burning) of the propellant. For small arms, pressure is measured in pounds per square inch (psi).
Gravity – the constant pressure of the earth on a projectile at a rate of about 9.8 meters per second squared, regardless of the projectile’s weight, shape or velocity. Commonly referred to as bullet drop, gravity causes the projectile to drop from the line of departure. Soldiers must understand the effects of gravity on the projectile when zeroing as well as how it applies to determining the appropriate hold-off at ranges beyond the zero distance.
Drag (air resistance) – the friction that slows the projectile down while moving through the air. Drag begins immediately upon the projectile exiting the barrel (shot exit). It slows the projectile’s velocity over time, and is most pronounced at extended ranges. Each round has a ballistic coefficient (BC) that is a measurement of the projectile’s ability to minimize the effects of air resistance (drag) during flight.
Trajectory – the path of flight that the projectile takes upon shot exit over time. For the purposes of this manual, the trajectory ends at the point of impact.
Wind – has the greatest variable effect on ballistic trajectories. The effects of wind on a projectile are most noticeable in three key areas between half and two-thirds the distance to the target:
Time (T) – the amount of time the projectile is exposed to the wind along the trajectory. The greater the range to target, the greater time the projectile is exposed to the wind’s effects.
Direction – the direction of the wind in relation to the axis of the bore. This determines the direction of drift of the projectile that should be compensated.
Velocity (V) –the speed of the wind during the projectile’s trajectory to the target. Variables in the overall wind velocity affecting a change to the ballistic trajectory include sustained rate of the wind and gust spikes in velocity.
The pictures illustrate how these terms interact to create the shot path of the projectile. Keep in mind these interact over a period of milliseconds to seconds.
So to sum up, we’ve defined several terms that apply to external ballistics. Next week, we will discuss how these relate to the application of holds for the Paratrooper.