Ballistics 101

by Tom Kelley

As I was rereading some old articles and coding some others up for posting on the website, it occurred to me that I have, over the years, used more than a few ballistics terms without explaining them as good as I could or should have. Therefore, let me take the time this month to discuss the science of ballistics and the terms that I and even learned ballisticians use.

Ballistics is defined in the Second College Edition of the Webster's New World Dictionary of the American Language as "the science dealing with the motion and impact of projectiles ... ," and "the study of effects of the firing on a firearm or bullet, cartridge, etc."[pg 107]

For skirmishing purposes, we call these two branches of the science of ballistics "external ballistics," which deals the flight, speed and energy of the projectile, and "internal ballistics," which deals with the combustion, friction and pressure within the fired arm.

Most discussions of ballistics regarding skirmishing deal with the external ballistics of a particular "load", a specific combination of projectile, powder and maybe some sort of wadding. However, internal ballistics also play an important part in the total ballistics of a load. The decision to use FF or FFF powder for a load is the most frequent exposure to internal ballistics that muzzleloading skirmishers encounter, but a pitted bore, an undersized projectile or the manufacturer of the percussion cap can also affect the internal ballistics of any load. Revolver shooters know that the cylinder gap, the amount of space between the cylinder and barrel, has a large impact on the internal ballistics of the revolver load they develop.

There are several terms used in discussing external ballistics that the reader should become familiar with. The most frequently used term from the realm of external ballistics is muzzle velocity. Muzzle velocity is the speed at which the projectile exits the barrel, and to be considered accurate should be the average of at least 10 repeated shots with the load being measured. Muzzle velocity is expressed at the rate of feet per second (fps) in American articles, but is usually expressed as meters per second in English, Canadian and European articles. Muzzle velocity is usually measured at a distance of 4 to 10 feet from the muzzle of blackpowder arms because of the large amount of material that is expelled along with the projectile in the ignition of these arms. We call it "muzzle" velocity because that is as close to the muzzle as we can get without wearing out the chronograph screens every time we conduct a measurement. The term velocity refers to the speed of the projectile, which is a variable that decreases the further you measure from the muzzle. Around Back Creek, we often use the term velocity when we really are discussing the muzzle velocity of the load being discussed.

Trajectory is another important external ballistic term, and refers to the path of the projectile from the muzzle to impact with the ground. As participants in a sport where marksmanship and group size are crucial, trajectory is the most fundamental concept of external ballistics. Even if a skirmisher doesn't know how fast his projectile is going, he or she must know where it is going. Trajectory is linked to velocity, or more correctly muzzle velocity, because the faster the muzzle velocity of a projectile, the flatter the trajectory of that projectile in flight.

Trajectory at skirmishing velocities can best be explained by the classic example of a household water hose with an old time nozzle on it. You could adjust the nozzle until a small, powerful stream was emitting from the nozzle. The first couple of feet of the stream could almost be considered "flat." The water steam arched an inch or two in the middle of the first six feet of the stream, but the water six feet from the "muzzle" of the nozzle was about the same height as the water coming from the nozzle when the nozzle was positioned horizontally. Past the point where the water was the same height, however, the water stream drops drastically, eventually falling almost straight down where the stream strikes the ground. A subsonic skirmishing round has a velocity path quite similar to the water stream from the nozzle. The flat portion of the trajectory is usually about 50 to 75 yards, then the path drops drastically as velocity rapidly deteriorates.

When velocity is discussed concerning loads and load development, two other terms are key to understanding the data. Since the velocity of the load is reported as an average, the spread of the individual data elements is of paramount importance. This spread is reported as the standard deviation of the data. To compute the standard deviation, first the average of the data is calculated. Then, each measurement is analyzed and recorded in distance from the calculated average of the aggregate data. These differences, or deviations, are than averaged to compute the standard deviation of the data.

Another good measure of data dispersion is the Extreme Spread (E.S.)of the data. The lowest element is subtracted from the highest element to compute the E.S.

Lets look at two groups of muzzle velocity data as hypothetical results from a test of two proposed loads for a 3-band Musket.

Group A: 1150, 900, 850, 1075, 1025, 950,1075, 1250, 1100, 1200.

Group B: 1068, 950, 1025, 1150, 1032, 1100, 975, 1125, 1000, 1075.

Both of these groups have the same average muzzle velocity, 1050 fps. However, Group B has a standard deviation of 53.6, while group A has a standard deviation of 105. 0. Additionally, the Extreme Spread of Group A is 400 (1250 - 850), while the E.S. of the second group is one-half that, 200 (1150 - 950).

I like to say that Group B has "better math" than Group A. I favor loads that have low two digit standard deviations and low extreme spreads. I would be more enamored with the Group B load for that reason in this example.

A final consideration when researching ballistics is how well do you shoot that load. Understanding all the pressures, deviations and spreads will not help you if you shoot Group A with a 2" group and Group B looks like a buckshot load. Ballistics is an important consideration, but is mostly science. Results - broken birds and paper holes - are the ultimate factor in choosing your personal loads. Results are the application of the science.

Until the next time, promote responsible gun ownership, shoot safe and have fun.

2002 by Tom Kelley

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