External Ballistics Part II

 

For Walkthrough Wednesday, we are continuing our discussion on External Ballistics. Understanding External Ballistics will allow us to engage targets better at varying distances by knowing where it is in relation to our line of sight or more commonly, our point of aim. When dealing with external ballistics there are a few misconceptions that must be cleared up and some basic facts that must be identified. Our reference for this discussion is TC 3-22.9, Change 1 Dated January 2017.

This begins when the projectile exits the bore. As soon as the projectile exits the bore, velocity begins to drop, as there is no longer any force to speed up or maintain the speed of the round. Air drag immediately begins to effect the round, slowing it down. As the projectile ends its contact with the lands and grooves of the barrel, the gas-tight seal that was behind the projectile is broken.

The expanding gas behind the bullet exits along with the bullet, and is expelled around the bullet and bore. The bullet will not be stable when it exits the bore due to lack of constriction provided by the barrel and by the exiting gasses. The bullet will wobble in flight (precession) until gyroscopic action stabilizes it. This initial wobble is known as the Magnus Moment.

It is the brief period in a bullets flight when it is not stabilized. Due to gyroscopic stabilization, the bullet may not travel directly aligned with the path of flight. Instead, the bullet will rotate around its center of gravity. The Magnus Effect will stabilize the bullet and cause it to point toward (about one degree off) the target.

The rifling twist rate becomes a factor now that the bullet is in flight. The bullet must be stabilized to have uniform drag. If the bullet is wobbling in flight, each individual round will have a different path and accuracy will decrease.

The general rule of thumb for twist rate is, longer bullets (typically the heavier ones) need more stabilization. This translates into lighter projectiles generally getting less twist, and heavier getting more. The exception to this being tracers because of their length.

The optimal amount of twist for a 62-grain bullet (SS109 in M855) is one rotation in eight to nine inches (1:8, 1:9). The optimal amount of spin for the M856 tracer is 1:5 to 1:6 due to its longer length. The best compromise for both trajectories to be about the same was to go with a 1/7 twist for the service rifle. Although not optimal, it is acceptable.

 

Wind deflection is the most influential element in exterior ballistics. The Wind does not push the projectile causing the actual deflection. The bullet’s tip is influenced in the direction of the wind slightly, resulting in a gradual drift of the bullet in the direction of the wind. The effects of wind can be compensated for by the shooter provided they understand how the wind affects the projectile and the terminal point of impact. The elements of wind effects are—

The time the projectile is exposed to the wind (range).

The direction from which the wind is blowing.

The velocity of the wind on the projectile during flight.

Picture two helps illustrate how to determine the direction and value of the wind.

Before adjusting the sight to compensate for the wind, the Paratrooper must determine wind direction and velocity. He may use certain indicators to accomplish this. These are range flags, smoke, trees, grass, rain, and the sense of feel. However, the preferred method of determining wind direction and velocity is reading mirage. In most cases, wind direction can be determined simply by observing the indicators. Appendix C, TC 3-22.9 Change 1 lists indicators to look for when estimating the wind:

0 to 3 mph = Hardly felt, but smoke drifts.

3 to 5 mph = Felt lightly on the face.

5 to 8 mph = Keeps leaves in constant movement.

8 to 12 mph = Raises dust and loose paper.

12 to 15 mph = Causes small trees to sway.

Keep in mind, the wind blowing at the Paratroopers location may not be the same as the wind blowing on the way to the target.

 

Once the distance, direction, and velocity have been determined, Paratroopers can compensate for the effects of wind. There are three basic methods of determining the appropriate hold-off to adjust for excessive wind; using the wind formula, wind estimation, or referencing a generalized ballistic windage chart.

Once the range to target and wind speed are known, the formula below is used to determine drift. The output from the formula is in MOA. The final answer is rounded off to make the calculation quicker to perform. This formula will allow the Paratrooper to adjust for the distance that the wind displaces his projectile. Picture three illustrates this for us.

The ballistics chart shows the wind drift in inches at ranges from 100 meters – 300 meters and wind speeds up to 20 mph. The data from the 100-m (meter) line shows that even in a 20-mph wind there is very little deflection of the round. At 300 meters, it can be seen that the same 20-mph wind will blow the bullet 26 inches. This illustrates the fact that the bullet is effected more by the wind the further it starts out from the target.

So to sum up, we’ve discussed what happens when the Projectile exits the muzzle in flight to its target. We discussed what the three elements of wind effects are. And we discussed the formulas that can be used to determine holds for the M855 projectile. Next week, we will continue our discussion on External ballistics, as we discuss how wind is affected by the battlespace.

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