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Determine Trajectory

All objects fall, including fast moving pellets! This means that when you shoot an airgun, you need to take several factors into consideration to hit your target. This is a short introduction to pellet trajectory.


Determine your "kill-zone"

A kill-zone is not necessarily linked to killing animals. It is simply a term that defines the radius of a circle in which a hit is "acceptable". For example, if you are plinking with soup cans, your kill-zone can have a generous radius of 20mm or so. If you are shooting field target, then you have to find the kill-zone of the target. If you hunt, you have to study the anatomy of animals and find out the size of zones where you can make humane kills.

Determine your point-blank range

The point-blank range is a range of distance in which you do not need to adjust elevation of a scope for a determine kill-zone. Of course, we all want the point-blank range to be "infinite"! In reality, you have to choose based on the majority of shots that you think you'll be making.

The physics
Pellet Drop

Earth's gravity is a constant. How much a pellet drops can be determined by how much time it has in flight or free fall. It makes no difference whether you have dropping a pellet from your hand, or a pellet is in flight after getting shot from an airgun. Note that the distance dropped is proportional to the square of amount of time.

Ballistic Coefficient (BC)

BC is a number that characterize how a pellet is slowed down by air. A larger BC means a pellet is less affected by air.

While BC is far from a hard science like gravity pull, it is still a useful component. A pellet with a larger BC does not get slowed down as much over distance, nor does it get affected as much by cross wind.

The draw back, however, is that pellets with better BCs are typically heavier pellets.

Pellet Mass

Pellet mass is measured in grains. Small caliber pellets like .177 ones typically weigh 7 to 8 grains, but you can find heavy ones at about 13 grains. Medium caliber pellets like .22 ones typically weigh 13 to 14 grains, but heavier one can exceed 20 grains. Big caliber pellets like .25 ones can weigh over 30 grains, averaging around 20 grains.

The mass of a pellet affects a pellet's muzzle velocity. The heavier a pellet, the slower it shoots out of the barrel. However, in general, heavier pellets have better BCs as discussed before.

Keep in mind that if you have a spring piston airgun, you need to choose pellets of the right mass. Otherwise, you risk reducing the lifespan of the spring (using pellets that are too heavy) or the piston seal (using pellets that are too light). 

Scope height

How high is your scope compared to the bore line? Some guns (bullpup configuration) require a higher scope mount, while others can have the scope mounted closer to the bore. Besides the style of stock, another factor can impact scope height: scope objective lens diameter. The smallest scopes have a 20mm diameter, which allows a very low scope mount. However, most scopes that have adjustable objectives (AO) have diameters ranging from a humble 32mm to a huge 60mm. A scope with a wide objective lens requires more scope height. 

Putting everything together

For a point-blank range that is relatively close (9 to 25 yards), you can benefit from a combination of slower muzzle velocity and lower scope height. This typically translates to an airgun with less power (energy), heavier pellets, and/or a smaller scope with a lower mount.

For a far point-blank range, you can benefit from a combination of a high muzzle velocity and higher scope height. This typically translates to a more powerful airgun with more energy, lighter pellets, and/or a larger scope with a higher mount.

What if you need to shoot outside of your point-blank range? First, you can benefit from a rangefinder and an inclinometer (if you shoot at an angle up or down). Many laser rangefinders report both distance and angle. Second, you can use a ballistic calculator. There are Android and iOS ballistic calculator apps that you can install. Third, you can benefit from a scope with finger turned indexed elevation adjustment. Most scopes classified as "tactical" or "sniper" have big turrets for elevation and windage adjustments. However, some "hunter" type scopes may still have adjustments that require the use of a coin.

Using the information about your gun and data from an inclinometer, a ballistic calculator returns adjustments for elevation. Depending on what you are shooting, you may or may not have time to enter all that data and wait for an output. Practice is the key! Some people also keep a cheat sheet in the form of a table to look up elevation adjustments in clicks given distance and angle.

You may find some free programs quite useful in determining the trajectory of pellets. Chairgun is a program published by Hawke Optics (same company that makes scopes), and it has a rich set of features to help analyze trajectory of pellets. It is highly recommended as a tool to help evaluate different hypothetical configurations.Type your paragraph here.