Air gun accuracy

Things have been very dead around here lately so here is some food for thought about air rifle accuracy. I hope this information will be useful for you if you are wondering why and how your rifle doesn't shoot how you expect it to and some possible reasons why.

Im going to start with the basics of accuracy and then get a bit more technical with pellet types and barrel twist rates and so forth. 

Part 1

When we think of accuracy in relation to our air rifles we think of punching several tiny little nice tight 5 or 10 shot groups on paper at some fairly good distances. But that is not really what accuracy is. Accuracy in its simplest sense, is nothing more than consistency. the more consistence, the more accurate you will be.

To put it as simple as possible. Aair rifle accuracy is the ability for our rifle to launch a pellet at lets say for arguments sake through our PCP at 850 FPS and have it arrive at our target at say 650 FPS one would think that if every shot we took did exactly that our pellet would be capable of shooting through the same hole shot after shot after shot but how often does this happen?. A lot of shooters assume if every pellet leaves the muzzle at exactly the same velocity every pellet will hit the target at the same terminal velocity but it really doesn't work like that. If it did, we would have absolute accuracy. Sure, we get small groups at long ranges; but why and what are the real reasons for this? Better yet what causes a seemly perfect pellet to be the one nasty flier in the group? Why do all the pellets cluster together, on the target; but, one has to always ruin things?

Im sure most of you out there have all heard this next story, or we have experienced it first hand. We have two guns that were made by the same gun maker. They are the exact same make and model, but one will shoot tiny groups with pellet X; while, the other shoots the same pellet all over the place. Why does this happen?. To fully understand this we must first understand what really happens to our pellet between the time we put it in the barrel and it hits the target. To understand accuracy, we must understand both internal and external ballistics.

We all know that high powered rifles are accurate at very long ranges and it is nothing for the best of them to shoot tiny groups at up and over 300+ meters. Then why does our airgun accuracy fall off at such close ranges? first have a look at 10 meter target guns and then field target guns. Accuracy for ten meter guns and Field Target range guns are not quite the same thing. First off ten meter guns shoot better at rather low velocities and shoot quite well or even more accurately with flat nosed wad cutter type pellets. These guns are great at what they do; but, as the range increases so does the need for power.

The ten meter match guns shoot what are probably the best made pellets you can get but. You will never see anyone winning a field target match with those pellets. The reason has to do with ballistics, or the ballistic coefficient of the pellet. The ballistic coefficient is the pellet's ability to maintain a higher velocity down range. Simply put, the higher the ballistic coefficient of the pellet, the further it will shoot accurately.

While the reason that high powered powder burner firearms can shoot accurately for such long ranges is partly do to the real high speed of the bullet but there is more to it than that. If you were to look at the design of a high powered rifle bullet you would see that there is a great deal of difference between its design, and that of our airgun pellet. a simple google search will show you.

If you take another look at a high power rifle bullet tells us that it is very aerodynamic but  take a even closer look at the design. The fact of the matter is that the back end of the bullet is much heavier than the sharp pointed front end of the bullet. If you were to shoot this bullet out of a smooth bore barrel it would be less accurate than an old rotten spud shot out of a 12 gauge shotgun at close range you would be lucky to hit a barn door at 20 meters with it.

So. What enables this high tech boat tail bullet to be aerodynamic and very accurate?, this is in the rifling of the barrel. These rifles usually have a fast twist rate; in actual fact they have a very fast twist rate. This causes the bullet to rotate very very fast. Without this fast rate of rotation, our high tech bullet would probably flip end over end.

Rotating our pellets through the use of rifled barrels helps give them stability as well so, they dont  flip end over end. One of the questions that we need to address is how fast do our pellets need to spin? Airgun barrels come in twist rates from as slow as 1:20 to 1:12 or in simple terms 1 turn in 20 inches to 1 turn in 12 inches. How do we decide which is best? And does it really matter? and what is the difference between them?

Without going into a bunch of math about which twist rate is best lets look at what works, how and why it works. The object of the rifling the barrel is to rotate the pellet and what we are really interested in is the pellets rotation rate. To understand rotation rate, we need to look at some math. Rotation Rate = (12 / Twist Rate) * Muzzle Velocity, in feet per-second. then times it by 60 The number 12 comes from 1 turn in a foot, or 12 inches; since we are using feet per-second. This gives our 150 grain 30 caliber rifle bullet traveling at 2900 feet per second a rotation rate of 174,000 revolutions per-minute, or 2900 revolutions per-second. Thats pretty insane.

Now It doesnt take a rocket scientist to work out that the faster we shoot our pellet/bullet the faster it will be rotating. if we were to take a .22 14grain pellet and shoot it at 800fps with a barrel twist rate of 1:12 our pellet will be spinning at 48000 feet per minute or 800 revs per second. 

However, since the actual rate of rotation is controlled by both velocity and twist rate these twist rates are not really that far off the mark as they should be. However, airgun pellets have bigger problems built into their design.

Sometimes when bullets are not properly made, they can suffer from static or dynamic unbalance. This is usually caused by the bullet having a hollow spot or spots inside. The key word here is unbalance. When a bullet is unbalanced; it quickly loses stability and accuracy goes out the window.

An airgun pellet looks more like a shuttlecock than a bullet and it has a great deal of air space around it and inside when it is flying through the air, when compared to our high tech bullet. This lends itself to the pellet's weight not being perpendicular or centered with its line of travel. In any real sense this design leaves itself open to static and dynamic unbalance and again the key word here is unbalance and any lack of balance can and will cause our pellet to wobble. That is to say, it will not travel in a very aerodynamic fashion.

Aerodynamic unbalance can also be caused by the rifling in the barrel what can happen is that coarse rifling will cause tiny chunks of lead to be pushed outward giving the head of our pellet a fan blade like appearance. Needless to say, that this is not good for the aerodynamics of our pellet. While this fan blade type of deformation is rare; all of the barrels with coarse rifling tend to do this to some degree. However, if you get rid of the rifling; you would also get rid of accuracy.