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Ballistics Calculators

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Author Bill Clarke


The serious airgun shooter wants to know and understand everything about accuracy. The ultimate accuracy of an airgun involves the technical specifications of the pellet being shoot, the speed that pellet travels at, and the distance from the point of impact.

Although the study of ballistics, regarding the performance of airguns and airgun pellets, can be complicated and involve many tedious calculations - much of the everyday information is easily available here in the Internet.



LEARNING ABOUT BALLISTICS

Background explanations of airgun ballistics, deffinitions, and examples are given in Airgun Ballistics from Beeman. It is a must read uncomplicated and very informative article.

THE BALLISTIC COEFFICIENT an article describing the ballistic coefficient as posted to Pneumatic Pnews by Randy Mitchell and written by: Der Luftmeister

If I am shooting my airgun at targets placed no farther than 10 meters away, I worry only about the accuracy of the pellet I am using. However, when I am roving with an airgun, I may encounter targets of opportunity at ranges from a few feet to as far as 50 yards or more. In that case, my concern is that the pellet retain its velocity to the greatest extent possible. The result is a trajectory that has the least curvature which in turn makes it easier to hit nearer the point of aim. It also means that the retained energy is as high as possible which is important if the target is some animal that is to dispatched cleanly.

When it comes to being able to move through air with minimal velocity loss, not all pellets are created equal. The ballistic coefficient is simply a number (in almost all cases between 0 and 1) that gives an index of how well a projectile in flight resists the retardation by air. It is a sort of batting average of the projectile against the atmosphere. In baseball, the higher the batting average the greater the likelihood the batter will get a hit. In ballistics, the higher the ballistic coefficient, the greater the fraction of the velocity retained as the pellet passes through air.

Pointed bullets used in center fire rifles have ballistic coefficients as high as 0.400 or 0.500 in many cases. If two bullets of identical weight have ballistic coefficients of 0.400 and 0.440, the difference is 0.040 which is only 10 percent. The difference in retained velocity will not much different for the two bullets if they are fired at the same muzzle velocity.

Although there are exceptions, the ballistic coefficients of airgun pellets are usually in the range of 0.010 to 0.030. That means that pellets lose their velocity rapidly. When comparing a wadcutter pellet having a ballistic coefficient of 0.010 with a pointed pellet that has a ballistic coefficient of 0.020, the difference is only 0.010, but that is a 100 percent advantage for the pointed pellet! Let us illustrate with a specific example.

Suppose your favorite .177 caliber Luftgewehr (air rifle) fires a 7.9 grain wadcutter pellet having a ballistic coefficient of 0.010 with a muzzle velocity of 800 ft/sec. Calculations show that the pellet will arrive at a target 40 yards from the muzzle traveling at 483 ft/sec. Now suppose a pointed pellet having a ballistic coefficient of 0.020 is fired with the same muzzle velocity. According to calculations, the pointed pellet will arrive reach a target at 40 yards with a retained velocity of 620 ft/sec. That is a 28.4% greater velocity for the pointed pellet.

When it comes to retained energy, the difference is even more dramatic because energy varies as the square of the velocity. In one case the remaining energy is 4.09 ft lbs while in the other it is 6.74 ft lbs (a 65% advantage for the pointed pellet). If your quarry is a critter, this difference could mean the difference between success and failure.

A higher ballistic coefficient not only means greater retained energy, but also a less curved trajectory. Suppose the rifle is sighted in at 30 yards. If we take the two examples of pellets and velocities described above, calculations show that the wadcutter will strike almost 1.5 inches low at 40 yards while the pointed pellet will have a point of impact that is almost exactly 1 inch low. The difference is not great, but if the lethal zone on your target is only about an inch in diameter, flatter trajectory is a great asset.

As a an approximation, the ballistic coefficients of flat pointed pellets such as Hobby and Meisterkugeln are around 0.010 to 0.012. The values for most domed pellets are around 0.015 to 0.020. For example, in .177 the Crosman Premiers are 0.023 and 0.027 for the light and heavy versions, respectively. These are among the highest values for .177 pellets. The .22 caliber Crosman Premier has a ballistic coefficient of approximately 0.028 and in .20 caliber the value is almost 0.040. Crosman pellets have very shallow base cavities and solid construction in the head area so their ballistic coefficients tend to be rather high. The .22 caliber 21.1 grain Beeman Kodiak has a ballistic coefficient of about 0.036 and is of solid construction. Some other domed pellets that are of a more hollow design have lower ballistic coefficients.

A pointed pellet does not always assure a high ballistic coefficient. For example, the Dynamit Nobel SuperPoint has a value of only about 0.012 because of the way it is constructed.

So you want to use the pellet with the highest ballistic coefficient to shoot at pigeons at 50 yards, right? Well, the most important consideration is accuracy. It does not do any good to have more remaining energy if the pellet does not strike where you want it to. But if two pellets give equal accuracy, I select the one with higher ballistic coefficient if I anticipate taking shots at relatively long ranges.

Other calculators can be found at:
Airgun Expo, American Airguns, Airgun Helper and Air Rifle Pellet Database

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GNU Exterior Ballistics Computer  
A free ballistic calculator, suitable for airguns and firearms...


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http://sourceforge.net/project/showfiles.php?group_id=216428



Posted By: GordonMcKinney
20 March 2009 12:21


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