Guide to select your airgun

What do you need to consider when you are select an airgun? Here is a short list of items. If you are considering purchasing an airgun, your best option is to come in to the store and chat with Don.

Power plant

The power plant of an airgun is the mechanism that stores and delivers the energy to propel pellets. 

Piston. A piston airgun has a spring (or other springy mechanism) connected to a piston that travels in a cylinder. Cocking a spring piston airgun means compressing the spring via one of several methods. When the trigger is pulled, the piston is let loose, pushed by the compressed spring. This pushes the air in front of the pistol to the pellet and propels it out of the barrel.

Never dry fire (without a pellet) a spring piston airgun. Without a pellet to slow down the piston, it can hit the end of the cylinder hard enough to destroy seals.
Do not keep a spring piston airgun cocked for too long (more than several minutes). A spring gets fatigued when compressed for too long, therefore weakening an airgun. 
Any piston airgun has recoil. The recoil is the direct result of the piston moving in the cylinder and stopping abruptly. Unlike firearm, most piston airguns have a back-then-forth recoil. This can destroy scope designed for firearm (even ones intended for big bore!). Be sure to talk to Don asking him what scopes are suitable for piston airguns!
Subtypes of piston airguns (based on cocking methods):

Break barrel. This is the most common type. The barrel itself is the lever to compress the spring. Break barrel airguns are easy to service because the entire barrel is exposed when loading a pellet. One potential problem is barrel droop, meaning the barrel may not line up 100% straight after cocking, making the gun shoot lower.
Side lever. This is a less common type. The lever to compress the spring is on the side of the action. The main advantage of a side lever airgun is that the barrel is fixed and cannot droop like a break barrel airgun. The disadvantage is that loading pellets and barrel cleaning are both slightly more cumbersome.
Under lever. A variation of side lever where the cocking lever is under the action. These airguns look clean.

Single stroke pneumatic (SSP). An SSP airgun has a small built-in air tank. The air in the tank gets pressurized when the cocking action pushes a plunger through a cylinder with an one-way valve. When the airgun fires, all the air from the small tank is released through another valve into the breech, propelling the pellet forward in the barrel.

Because an SSP airgun relies on a single stroke, it tends to be weaker than other types of airgun.
There is no spring or piston in an SSP. As a result, there is little recoil. This is important in a match rifle or pistol.

Multi-stroke pneumatic. A multi-stroke pneumatic airgun is similar to an SSP airgun, except that it is designed to get pumped multiple times. This has several advantages. First, the power level can be controlled by the number of pumps. Second, assuming a user is strong enough, a multi-stroke pneumatic airgun can shoot stronger than an SSP airgun.

A multi-stroke has no recoil, like an SSP airgun.
Some multi-stroke pneumatic airgun takes up to 10 pumps to reach the maximum power level. This means quite a bit of exercise and time between shots!

Precharged pneumatic (PCP). A PCP airgun has a large air tank that is precharged to a pressure typically ranging from 1500psi to 3000psi. This means the shooter does not need to pump even once! Because a PCP air tank stores air at a pressure much higher than that of a SSP or multi-stroke pneumatic airgun, a PCP can also shoot at a high energy level.

A PCP airgun requires some source to charge the air tank. You can use a hand pump, but that can be difficult for some people, and impossible for lighter people. Most PCP airgun owners use a SCUBA tank to charge the air tank on the airgun.
A PCP airgun also has no recoil.

Carbon dioxide (CO2). A CO2 airgun uses a capsule that has liquid CO2 under high pressure. The CO2 capsule replaces an air tank as the source of compressed air. 12 oz. CO2 capsules are very inexpensive and compact.

The power of a CO2 airgun varies with capsule temperature. Capsule temperature itself depends on ambient temperature and frequency of shots. If you shoot a CO2 airgun in rapid succession, the CO2 capsule cools down, and the power level drops.
Many CO2-powered airguns (esp. pistols) are designed to resemble actual firearm versions. Some even have blow-back action.

Energy level

While some manufacturers use velocity (speed) as a gauge of airgun power, the proper measuring method is to use energy. Skipping physics concepts, energy is "work done". In the U.S., airgun energy level is measured in foot-pound, typically abbreviated to fpe (foot-pound energy) or ft-lb. One fpe is the energy required to lift a pound up one foot. 

Lethality, range, damage and penetration are all proportional to energy level, not velocity. A fast moving light pellet can do less damage compared to a slow moving heavier pellet. You can use a fpe calculator to compute the fpe based on pellet velocity (in fps, feet-per-second) and mass (in grains).

"Over-the-counter" airguns typically start at about 3 fpe, and can reach levels of about 30 fpe. Rare custom one-of-a-kind exotic big bore airguns can exceed 100 fpe. Some airguns let you vary the energy level. For example, the energy level of a multi-stroke pneumatic airgun depends on the number of strokes. Many PCPs offer an adjustment mechanism to vary energy level. Piston airguns typically cannot change the energy level.

How do airguns compare to firearm in terms of energy level? A .22 LR round, long considered a "plinker round" and "unsuitable for self defense" amongst firearm enthusiasts, has more than 100 fpe when shot from a rifle. "Man stopper" pistol rounds (.45 ACP) used by law enforcement can exceed 600 fpe. A typical .223 round has more than 1100 fpe. A 30-06 round approaches 3000 fpe.

How much energy do you need?

First, you need to think what is the maximum energy that is still safe. If you shoot at a range (such as the DGS airgun range!), this (the energy rating, not safety!) is obviously not an important consideration. Otherwise, do you have a proper backstop where you shoot? Can your backstop really stop pellets landing at the same spot? A 1/2 inch wooden board can stop a pellet at 4 fpe once, 2 or 3 repeated shots to the same spot will penetrate the wood and send pieces of pellets to whatever is behind the board! 5 to 6 fpe will penetrate both sides of a soup can (not a soda can). 6 to 7 fpe will penetrate a half-inch wooden board in one shot!

It is not always "the more the better". A powerful (high energy level) piston airgun has a lot of recoil, making it very difficult to cock and hold-sensitive. A weaker (low energy level) CO2 airgun is great for plinking because you get more shots from the same capsule. 

For plinking (casual shooting bottles, cans and etc.), even 3 fpe is enough. It doesn't take much to punch holes on paper or penetrate soda cans on both sides! 

For pesting, the required energy depends on the pest, pellet, distance and shot placement. Even 4-5 fpe of muzzle energy can be lethal to many rodents and pest birds with head shots within a range of 15 yards or so. Many people opt for more energy (up to 12 fpe or so) so that shot placement is not as important.

Caliber

The caliber of a gun is the diameter of the barrel. Commonly available airguns range from .177 inches to .25 inches. At a big box store, you can only find .177 and .22 caliber airguns. DGS carries airguns of .177, .22, .25, .30 and other calibers!

How do you choose a caliber, given that you have a choice? Remember, some airguns are only available in a certain caliber.

Because the barrel diameter determines the cross section area, it affects how much force (of compressed air) can act on a pellet when it accelerates in the barrel. This means the bigger the caliber, the easier to transfer energy to a pellet, and therefore the higher the energy rating. Given the same power plant, a big caliber airgun (.25) always have a higher energy rating compared to a small caliber version (.177).

There are some trade offs, however. A .25 pellet weighs typically 27 to 30 grains, compared to the typical 7 to 8 grains of a .177 pellet. At almost 400% the mass, a .25 pellet travels 50% slower than a .177 pellet given the same energy level. The impact is that the .25 pellet has more bullet/pellet drop and hence "arcs" more due to the slower velocity. The arcing (trajectory) means that a scope zeroed at 10 yards may need to be adjusted or compensated at other distances. With a flatter trajectory, a .177 gun has a wider "point-blank" (no need to adjust or compensate) range than a .25 gun. 

Experienced airgunners either adjust the elevation dial of a scope or choose a mil-dot scope to compensate for pellet drop at different distances. However, doing so requires knowing the distance and angle to a target, which makes "quick shots" very difficult.

In terms of cost of ownership, .177 pellets are generally very inexpensive, while .25 pellets are more costly. On the other hand, compared to firearm ammo, the cost of pellets is almost negligible. Most people do not use this as a criteria of deciding the caliber.

Another consideration is terminal velocity and potential damage/injury resulting from a stray pellet. The terminal velocity of a pellet (or any object) is the velocity that is constant as a pellet falls and cannot accelerate anymore due to atmospheric drag. The velocity is determined by the equation: Terminal V (fps) = 879 x sqrt(BC). A typical .177 pellet has a typical BC (ballistic coefficient) is about 0.020. As a result, its terminal velocity is about 124 fps. From this, you can compute the energy of a pellet at its terminal velocity Terminal KE (fpe) = 1.72 x grains x BC. A 7.9gr 0.177 pelllet with a BC of 0.020 has a terminal energy level of 0.27 fpe. Note that a pellet is likely to tumble as it slows down and reduce the BC to less than 0.01, which further reduces the terminal velocity and terminal KE.

Let us now apply the same calculations to a .25 pellet that has a BC of .031 and weighs 27 grains. It ends up with a terminal velocity of 155 fps and a terminal energy of 1.44 fpe. What does this mean?

Airsoft guns are required to stay at a level of 1J or below. 1J is about 0.74 fpe. This is because that is the recognized energy level where a projectile can break skin. A 7.9gr .177 pellet with a BC of 0.020 has a terminal energy of less than 0.4J, whereas a .25 pellet has a terminal energy of almost 2J. 

If the said .25 pellet is to be dropped from an airplane, it will break skin when it hits someone on the ground. On the other hand, the said .177 pellet is "relatively" safe, although it may still sting if it is to hit a person (about the same as a 0.2 gram Airsoft BB at 205 fps). This  is very serious because it means the .25 pellet cannot be safe no matter how it misses the intended target and is to hit a person! By comparison, the .177 pellet is relatively harmless at its terminal velocity.Type your paragraph here.

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