AIRGUN, CALIBER & PELLET SELECTION

[brihacharan]

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AIRGUN, CALIBER & PELLET SELECTION
by Tom Holzel - Article reprinted from BEEMAN Precision Air-gun Guide

Air-guns are mechanical devices that turn the stored potential energy of a coiled spring or compressed gas into kinetic (moving) energy, and transmit it to a projectile. However, the kinetic energy or "power"' of a gun is not measured directly - Rather it is the energy imparted to the projectile (pellet) that is measured. This pellet energy is the product of "velocity" and "weight" ("mass"), both of which are measured separately.

TARGET AIRGUNS:
Precision air-guns that are used exclusively for target shooting require only energy equal to 3 to 5 ft-lbs to speed a .177 caliber pellet through 10 meters of air. The speed component can be low, but the accuracy of a pellet must be exceedingly high, on the order of 0.04 center to center (c-t-c).

With target air-guns, repeatability of the power plant is a major aspect of high-quality. So are such accuracy related aspects of precision shooting as the character of bigger let-off (and its repeatability), adjustability of trigger pull and let-off and the adjustable fit of the air-gun to the hand and body.

The ergonomics of reloading the air-gun is also important to target shooters. This function should be as smooth and un-stressful as possible to keep concentration at a world-class level. Pellet insertion should be easy. CO2 and pre-charged air-guns eliminate one physically distracting aspect of target shooting, re-cocking that result in a smoother shooting process.

PELLET CALIBER & WEIGHT

1.CALIBER.

By far the largest number of air-guns sold, are in the 0.177 caliber and this is the pellet size of choice for nearly all target shooting, plinking and some small pest hunting. 0.177 offers the highest velocity for a given amount of air-gun energy, and results in the flattest trajectory.

However, the speed of sound--1080 fps at sea level at 32* F sets a practical upper limit on the energy with which a pellet can be propelled. Breaking the sound barrier results in a loud crack-just like a firearm and generally sends the pellet tumbling wildly. The 0.22 caliber has a large gain in pellet weight and size and is ideally suited for high-powered air rifles. The range of the .22 caliber is less and might be the choice if you owned a single air rifle.

2. PELLET TYPE
The single most important factor in choosing a pellet is to obtain one that is accurate in your air-gun! Only personal experimentation will let you discover the most effective pellet for your air-gun/target combination. Every air-gun varies slightly in the way it handles different pellet types. Since the accuracy of pellets themselves will vary slightly from batch to batch, it is wiser to buy adequate supply of pellets at one time than to buy in smaller quantities.

A light pellet will accelerate rapidly and leave the gun barrel at high speed. It's time in the barrel is the shortest, thus reducing the effects of an unsteady hold. The light pellet's time-to-target is also shortest so gravity can pull on it for only a split second. An accurate, very flat trajectory is the result.

Yet in some high-powered rifles, light pellets are ejected so rapidly they do not dwell long enough to get the full energy transfer of the decompressing charge of air. While the same rifle can propel an 8.6 grain pellet to 850 fps and a 14.2 grain pellet to 700 fps.

The speed of a heavier pellet is lower in the same air-gun versus a lighter pellet. And, because of its slower speed, a heavy pellet takes a longer time to get to the target; this gives gravity a longer time to pull it down.

Note that the drop of any pellet has nothing to do with its mass or weight--all pellets are pulled down by gravity at the same rate. The only thing that counts is how much time gravity has to do the pulling. It is only because heavy pellets take longer to get to the target that their trajectory is more bowed. A light-weight pellet traveling as slowly as a heavy weight would have an equally bowed trajectory

AIR RESISTANCE
Once clear of the barrel, another 'energy thesis' begins its work: air resistance. Air resistance increases with the cube of a pellet's speed: double the speed, and air resistance increases eight times! This means fast pellets lose energy more rapidly than slower pellets. Energy equals mass times velocity squared. Since the energy imparted to a pellet is about the same and a pellet doesn't lose mass, it can only make up for the different weight by changing velocity.

In fact fast, light pellets lose energy so rapidly that after 35 yards or so they can be traveling slower than heavy pellets. This is inconsequential in 10 meter target shooting, but it becomes a major disadvantage in hunting and field use.

WHAT PELLET SHOULD ONE USE?
Flat-nosed wad-cutters punch perfect holes in paper targets to aid in scoring and are required in competition. There is a slight effect of higher air resistance on accuracy due to the flat head & are un-measurable at 10-meter range, but do become noticeable at 35 meters and beyond.

Medium weight round nose pellets offers the best of weight and flat trajectory for medium-powered hunting rifles (12 to 15 ft lbs.). The closer you can normally get to your prey, the heavier the pellet you should use because heavy pellets penetrate much deeper and be less susceptible to wind deflection.

ACCURACY
Assuming an air-gun always exerts the same force on a pellet, the accuracy of a pellet's path--it's trajectory--is effected by three major (and some minor) factors:

1.CROWN
During its rush up the barrel, a pellet is constrained from going anywhere but straight forward. At the instant the pellet leaves the barrel, it is desirable that the barrel’s "crown" always presents exactly the same surface to the spinning tail of the pellet. Ideally, the barrel loses contact with the entire circumference of the tail of the pellet at the same instant so that the pellet is not tipped one way or the other. Tipping the pellet imparts a wobble, increasing the cross-sectional area through the air.

The wobbling pellet effectively increases its caliber causing more air resistance! This increased area will slow the pellet down more quickly than if it pierced the air perfectly head-on. Many air-guns are "button choked" at the crown of the barrel to assure a perfectly uniform grip on the pellet circumference the instant it leaves the barrel. Others are exquisitely detailed to assure a perfect pellet release. Be sure not to damage the crown of an air-gun barrel.

2.UNIFORMITY
Spinning is what keeps a pellet from tumbling, and keeps it facing directly forward as it bores through the air. Air gun guru ROBERT BEEMAN has pointed out that if a pellet is the slightest bit unbalanced as it spins, the centrifugal wobble will lurch the pellet off course the instant it leaves the barrel. The direction of that lurch will vary with every shot depending on which direction the excessive mass of the pellet is pointing at the instant it is released from the "hold" of the barrel. This little appreciated factor is one reason why he insists on paying attention to pellet quality control.

3.WIND
Supersonic bullets are more deflected by wind the slower they go. Surprisingly, sub-sonic air-gun pellets are less deflected by wind the slower they go, but this seeming anomaly is due to the higher weight of the slower pellet, assuming in this comparison that both are shot out of the same air-gun.

For any air-gun, a pellet with a higher "ballistic coefficient" will be less deflected by wind. Generally, heavy pellets have a higher ballistic coefficient than lighter ones.

AIRGUN SHOT REPEATABILITY
An air-gun does not exert the same force from shot to shot. Without such high repeatability, even excellent pellets will not be able to do their job. Average pellets may safely exhibit a weight differential of up to a few percent in any one tin. When both pellets and air rifle vary randomly, you will find yourself hitting the target perfectly (on some shots) and miss it completely on others.

Every air-gun user should obtain a tin of good quality pellets to determine just what the air-gun is capable of (bench rest), and what the shooter is capable of (hand held). You can use mid-grade pellets for practice, but always use the most accurate pellets you can find for competition and actual hunting.

CONCLUSION
Air-gun shooters should use this guide to help select the three or four different pellet types that seem closest to answering their own shooting needs. Then shoot these pellets for accuracy to get an idea of how well each type behaves in your own air-guns. Shoot into bars of Glycerin soap at field ranges to learn how well different pellets penetrate and expand. There is no substitute for this personal testing which will quickly lead to the selection of the most effective pellet type for your own particular use--and boost your accuracy and shooting satisfaction to new highs.
HAPPY SHOOTING!!!
Briha

[anilrana2168]

Nice and informative article