HUNTING WITH AIRGUNS
by Jim House
Chapter 6: Airgun ballistics.
A hunter who goes after small game and pests with an air rifle chooses to accept a handicap compared to the hunter who uses a firearm. As a result, a knowledge of the ballistic performance of an air rifle is essential for its effective use, and the hunter using an air rifle needs to be more knowledgeable about ballistics than one who uses a firearm. A rifle chambered for the .22 long rifle cartridge fires a 40-grain bullet at approximately 1200 ft/sec. A powerful .22 caliber air rifle such as a Benjamin Discovery or Marauder fires a 14.3 grain pellet with a muzzle velocity of approximately 900 ft/sec. The firearm generates a muzzle energy of approximately 130 ft lbs of energy at the muzzle whereas that of the air rifle is only about 26 ft lbs. One frequently hears the expression describing an air rifle as “shoots as hard as a .22”, but the firearm is much more powerful than any air rifle except perhaps some of the big bore .357 precharged pneumatics.
Pellet velocity is important, but it is by no means the only factor to consider. It should be remembered that the advertised velocity for any air rifle is usually based on the velocity obtained with a very light pellet and even then may be a generous estimate. For example, in .177 caliber a very light pellet might weigh only 6.7 grains while one of normal weight is about 7.9 grains and a heavy type might weigh as much as 10 grains. The velocity produced with pellets of the heavier weights might be as much as 100-150 ft/sec lower than with the light pellet. As a result, a break action rifle advertised to give 1000 ft/sec might give only 875-900 ft/sec with pellets of normal weight.
In addition to the power factor, there is the difference in the trajectory of the projectile. Pellets used in air rifles do not have the aerodynamic efficiency of bullets used in firearms and, consequently, they lose velocity rapidly. Even the bullets used in .22 rimfire rifles do not have the ability to penetrate air nearly as well as the streamlined bullets used in center fire rifles. The ability of a projectile to retain its velocity when passing through air is reflected by a number known as the ballistic coefficient. The higher the ballistic coefficient, the less air resistance retards the motion of the projectile. For a relatively efficient .22 caliber pellet such as the Crosman Premier, the ballistic coefficient is about 0.028, but the typical 40-grain bullet of a .22 long rifle cartridge has a ballistic coefficient of approximately 0.125. The result is that not only does a pellet fired from an air rifle have a muzzle velocity lower than that of even a bullet fired from a .22 rimfire, it loses its velocity much more rapidly. All of this means that the path of the pellet involves a lot of curvature and it is more easily blown off course by wind. An air rifle does not have the effective range of even a .22 rimfire, and the hunter who uses an air rifle must be aware of this limitation. Figure 1 shows how airgun pellets typically lose velocity over distance with the examples shown having muzzle velocities of 800 and 600 ft/sec.
Figure 1. The variation in pellet velocity with range.
A .177 caliber air rifle has one advantage because higher velocity results in a trajectory with less curvature. This means that it may be somewhat easier to hit a small target than it would be with a rifle of larger caliber, but the pellets of larger diameter hit harder. In any case, the lethal zone on some small species may be only an inch or an inch and a half in diameter. Under field conditions, it is often difficult to hit a target of this size even at a distance of 40-50 yards. Therefore, the accuracy of the air rifle may be less important than the skill of the shooter. When after small game, I always opt for the larger caliber because of the added punch of heavier pellets. In most cases, the larger, heavier pellets retain velocity more effectively which also is an advantage.
As soon as a pellet or any other projectile leaves the muzzle, the force of gravity starts to work on it pulling it downward. This results in a projectile following a curved path known as the trajectory. Sighting involves looking in a straight line. Therefore, there is some discrepancy between the line of sight and the path of the projectile. The sights are placed on top of the barrel so the bore is actually below the line of sight. If a scope sight is utilized, the bore may be as much as an inch and a half below the line of sight. In order to make the sighting error as small as possible, the sights are arranged so that they are aligned on the target with the bore pointing slightly upward. In that way, the projectile starts out moving slightly upward relative to the line of sight. The pellet rises (because it is fired slightly upward) to meet the line of sight. The distance at which the projectile meets the line of sight is the distance at which the rifle is “sighted in.” In most cases, the trajectory crosses the line of sight as the pellet rises and then crosses it again as the pellet continues on its downward path. The height of the bullet path above the line of sight at the midpoint of the sight in distance is referred to as the midrange trajectory. Thus, the rifle is actually sighted in at two distances, the longer of which is generally considered to be the distance for which the rifle is sighted in. This situation is illustrated in Figure 2 which shows typical trajectories for identical pellets fired at 600 ft/sec and 800 ft/sec.
Figure 2. The paths of identical pellets fired at 800 and 600 ft/sec when sighted in at 30 yards.
In both cases, the rifles are sighted in at a range of 30 yards. Note that the path of the pellet crosses the line of sight in two places one of which is 30 yards. But the pellet also crosses the line of sight at a shorter distance that depends on the velocity of the projectile. Note also that the heights of the trajectories above the line of sight at a range of 15 or 20 yards differ with the slower moving pellet having to rise higher in order to cross the line of sight at a distance of 30 yards, the sight in range. We say that the faster moving pellet shoots “flatter” meaning that its path has less curvature than one traveling slower. Note also that when the rifle is sighted in at a distance of 30 yards, the path of the pellet at 50 yards is about 6.6 inches low when the muzzle velocity is 600 ft/sec but only about 3.2 inches below the line of sight if the muzzle velocity is 800 ft/sec. It is readily apparent that a high muzzle velocity makes it easier to hit small objects at longer ranges.
As has been mentioned, the higher the ballistic coefficient the less rapidly the pellet loses velocity. That principle is demonstrated graphically by considering two pellets that have ballistic coefficients of 0.010 and 0.020 when both leave the muzzle at 900 ft/sec. Figure 3 shows the remaining velocity of such pellets over a range of 50 yds.
Figure 3. The loss of velocity for pellets having ballistic coefficients of 0.010 and 0.020.
It should be apparent that the pellet having a ballistic coefficient of 0.020 has a remaining velocity at a distance of 50 yards that is about 160 ft/sec higher than that of the less efficient pellet. The kinetic energy is a function of the square of the velocity so there is a very large difference in the remaining energy of the pellets. This is of vital concern to the hunter and illustrates that a pellet that has a high ballistic coefficient is generally a better choice but only if it gives excellent accuracy.
When you perform some physical activity at high altitude, you note how quickly you become winded. The reason is that at high altitude the atmosphere is “thinner” because the density of the air is less. When you inhale, you may fill your lungs, but it is with air at a lower pressure and not as much oxygen is present. The same phenomenon operates on some types of airguns. For example, cocking a break action rifle moves a piston to the rear causing air to be drawn into a chamber. When the rifle is discharged, the piston moves forward to compress the air behind the pellet. However, the volume of the compression chamber inside the rifle is fixed. When the rifle is cocked at high altitude, the piston moves to the rear but less air is drawn into the compression chamber due to the “thinner” atmosphere. When the rifle is fired, there is less air to be compressed behind the pellet and that causes the velocity to be lower than it is at low altitude. In fact, my tests have shown that at an altitude of 5,500 ft the velocity is generally about 6-7% lower than when the same rifle is fired with the same pellet at an altitude of a few hundred feet. If the altitude is around 8,000 ft, I found by testing several air rifles that the velocity is reduced by approximately 11-12% from what it is at low elevation. This is one aspect of airgun performance that is often overlooked. However, the lower muzzle velocity is somewhat compensated for by the lower air resistance as the pellet moves toward the target. As a result, pellet velocity does not decrease as rapidly as it does at lower altitude.
Break action air rifles are not the only types affected in this way. Each stroke of a multi-pump rifle draws in less air at high altitude so after a series of pumps there is less air in the reservoir than if the rifle were pumped at low altitude. As a result of the lower pressure of the compressed air, the velocity is lower by about 9-10% at an elevation of 8,000 ft. however, one or two additional pump strokes can used to overcome this phenomenon.
The situation is different for precharged pneumatic rifles. The reason is that such rifles are pumped until the pressure in the reservoir is at the desired level (usually 2000-3000 psi) regardless of the altitude. What is different is the number of pump strokes required to reach that pressure. The result is that a precharged pneumatic rifle that has a given pressure in the reservoir produces the same velocity regardless of the altitude. Rifles powered by CO2 are similar. The pressure inside the CO2 cylinder depends on the temperature, but not the external pressure. Therefore, as long as the temperature is the same there is no effect of altitude on the velocity.
The point of understanding ballistics as applied to airguns is that the hunter should know the limitations imposed by power, accuracy, and skill. Do not attempt to stretch the range of an airgun beyond what is certain. A .25 PCP may be an effective rifle for use on squirrels at 50 yards if the skill of the shooter and the accuracy of the rifle are sufficient. That is not the case of a .177 rifle that shoots lightweight pellets that lose velocity rapidly. Know your limits and those of your rifle and hunt within them.