ALL ABOUT VELOCITY
Velocity is defined as the change in distance with time. In other words, it is how far an object goes in a unit of time. Velocity is expressed as distance divided by time so we have units such as miles per hour (mi/hr), feet per second (ft/sec), or meters per second (m/sec). A baseball pitcher may throw a baseball at 95 miles per hour, but that velocity is also 139 ft/sec or 42 m/sec. The units used to describe the velocity of an object give different numerical values, but the velocity of the baseball is the same.
Velocities of projectiles fired from airguns are usually specified in ft/sec or m/sec. In this discussion, velocity will be expressed in ft/sec. Low powered airguns launch projectiles at 250-300 ft/sec while high powered break action models may drive pellets at 1,000 ft/sec or more.
It is the velocity of the projectiles fired from airguns that defines some uses of the guns. An air rifle that fires pellets at 1,000 ft/sec is a high powered model. Such a rifle is suitable for all types of plinking as well as pest control. The multi-pump Benjamin 392 and 397 and Sheridan Blue Streak and Silver Streak are also suitable for all types of plinking as well as pest control. On the other hand, lower powered airguns are suitable for target shooting and plinking.
Because of differences in weight, shape, and diameter, pellets do not have the same ability to retain their velocity when passing through air. Some are more streamlined and aerodynamically shaped than others. This is not a difficult concept to grasp when you realize that some cars are sleek and others are boxy. Sleek cars pass through air with less resistance than do boxy models. It would be convenient to have an index, a number, that expresses the ability of a projectile to pass through air. Then, by comparing these numbers we would be able to tell which pellets would retain their velocity better.
Such an index already exists. It is known as the ballistic coefficient which is simply a number. Think of it as a batting average. The higher the batting average, the greater the probability that the batter will get a hit in any time at bat. The higher the ballistic coefficient of a pellet, the better it retains its velocity when passing through air. Pellets generally have ballistic coefficients that vary from 0.010 to about 0.035. One of the factors that determines the ballistic coefficient of a pellet is the shape of the front end. Wadcutter pellets have flat noses so they cut a clean, round hole in a paper target. The flat point reduces the ability of such a pellet to pass through air so the ballistic coefficient is low (usually 0.010 to 0.012). On the other hand, pellets having a pointed or round nose are more streamlined and penetrate air much better so they have ballistic coefficients of approximately 0.020-0.030.
Suppose pellets having ballistic coefficients of 0.010 and 0.020 are fired at 700 ft/sec from the same airgun. How do the velocities differ after the pellets leave the muzzle? The accompanying figure can be used to answer this question. Although there is not much difference when the range is short, note how much faster the pellet having the higher ballistic coefficient will be traveling at longer ranges.
Crosman pellets have rather shallow cavities in their bases compared to those produced by most other manufacturers. This results in a solid front section and less air drag. One pellet that has a high ballistic coefficient is the Crosman Premier which is a domed pellet that is available in .177 (7.9 and 10.5 grains), .20 (14.3 grains), and .22 (14.3 grains). These pellets and other Crosman domed and pointed pellets are excellent choices when downrange velocity and energy are important (as in pest control and silhouette shooting).
As a projectile moves through air, it loses velocity. Air is a mixture of gases and it requires energy to move the molecules out of the path of the projectiles. That energy comes from the projectile so it loses energy (and the velocity decreases) on its way to the target. Generally, the faster the projectile is moving, the greater the air resistance and the faster the projectile loses its velocity. A certain pellet having a ballistic coefficient of 0.015 fired with a muzzle velocity of 500 ft/sec will arrive a target 30 yards away traveling at 386 ft/sec (a loss of 22.8%). The same pellet fired at 950 ft/sec will arrive at 30 yards with a velocity of 715 ft/sec (a loss of 24.7%). Of course the faster a pellet is fired, the greater its velocity will be at any given distance from the muzzle, but the percent of velocity lost is slightly greater the higher muzzle velocity.
One thing to keep in mind is that the projectiles fired by airguns do not hold their velocity very well compared to bullets fired from firearms. A pellet that leaves the muzzle of an airgun at 800 ft/sec may be traveling at only 600 ft/sec after it has traveled a distance of 30 yards. As a result, an airgun selected for pest control should be a model that has ample power at the distance where pests will be encountered.
In order to show the effect of the muzzle velocity, let us consider two cases in which the same type of .177 caliber pellet having a ballistic coefficient of 0.015 is used. In the first case, we will assume that the pellet is launched from the muzzle at 600 ft/sec and in the second case the muzzle velocity is 800 ft/sec. In each case, we will assume that the rifle is sighted in to hit the point of aim at 30 yards. The accompanying figure shows the path of the pellet in each case. Note how much less curvature there is to the path of the pellet fired at the higher velocity. This is one of the advantages of using an airgun that produces higher velocity because less curvature in the path of the pellet means that it is easier to hit a small target.
For many airgun shooters, there is no reason to be concerned about pellet velocity. That paper target or pop can may respond in a predictable way when it is hit regardless of the velocity of the pellet. But for the shooter who wants to knock over a metal silhouette at 40 yards or eliminate pigeons from a barn, velocity is an important factor in achieving success. The type of airgun selected, the ammunition used, and the sighting equipment on the gun are extremely important, but these topics are covered in detail in other articles in this series.