Ballistic Harmonic Spiral (Pellet Spiral)

Pellet Spiral is the phenomenon occasionally observed through a telescopic sight of “recoilless” air rifles where the pellet is seen to spiral towards the target

The observation sometimes seen as counter intuitive, often considered an impossibility and occasionally attributed to movement in the rifle after the pellet has left the muzzle – rather than the trajectory of the pellet its self.

Pellet Spiral is in fact a real effect associated with spin-stabilized bullets acted on by a moment of force known as the “Magnus Moment” (See Google for detailed info)  and in complete agreement with the laws of physics!

Pellet spiral may not be observed in every pellet fired from a rifle.  This often contributes to the perpetuated myth that it’s not a real effect or is an optical illusion.  This is misleading

Pellet spiral is real enough and one of the reasons that it is not seen in every shot is mainly that a number of specific physical effects must “align” or “harmonise” (technically speaking - a “harmonic” being set up) for the spiral to occur. 

Key factors include

1.       Yaw angle and processional speed

2.       Velocity

3.       Spin rate

4.       Bullet geometry

When specific effects do harmonise in such a way that a type of harmonic is set up then pellet spiral may occur. 

Before discussing the “harmonic” causing the pellet spiral it is important to understand what “yaw” is

A simple description is that a spin stabilised bullet with yaw is constantly and rotationally changing (as a factor if is rotational speed) the “direction it is facing” in relation to the “direction it is travelling”

Yaw can be visualised by imagining a spinning top with some BluTak stuck to its side then set spinning.  As you can imagine the spinning top spins out of balance or “wobbles”.  A pellet will also spin out of balance in flight or “wobble” just like an out of balance spinning top.  This is because no pellet is absolutely perfect on the small scale.  It may come as a surprise to some of us that pay a premium for top quality pellets only to discover that if we looked at them more closely under a relatively small magnification microscope or simply a good jewellers magnifying glass that there are several imperfections which have the potential to cause pellet out of balance effects.  (If a motor cars flywheel was as imperfect as our pellets are then they would shake the car to pieces in seconds – yet we expect our pellets to be perfect enough to “one hole group “at 20 meters or more!) Simply measuring the weight of pellets and selecting the ones within a narrow average weight is established good practice for promoting consistency and accuracy.  However , this is not the complete story!  Ideally each pellet should be selected on its rotational balance.  However “in the real world” it’s obviously not practical to measure each pellets rotational balance (there is just no such commercially available equipment available to do this economically) and anyway if it were economically possible there would be few people who would know to do so anyway.   Most of us simply check the pellet visually for imperfections – which is good enough for most purposes.  However this does not rule out every imperfection, particularly those within the pellets skirt/interior.  So neglecting this check can contribute (but not exclusively lead) to “flyers”

Getting back to the point about yaw.  If a pallet is yawing then, again in simple terms, it is trying to go in a direction slightly contrary to the direction of travel.  (This is similar to the analogy of trying to throw a paper aeroplane forwards while aiming it slightly to one side by 20 degrees.  It will try to fly in the direction the plane is pointing rather then the direction it is thrown. )  If the yaw of the pellet is a circular motion (i.e. constantly changing due to the spin imparted to it by the rifling) then the direction that the pellet is trying to go in is constantly changing in a circular fashion or putting in another way spiralling around the intended direction of travel.  It is not quite as simple as this (and the likely subject of the next article) but is a fair approximation.  There are actually several forces acting on a pellet in flight and the spiral rate will always be a division/factor of the  pellet spin rate/frequency (imparted by the barrel rifling) depending of the complex interaction of the forces acting on the pellet (Including the Magnus Moment which is a spiral inducing vector in spin stabilised bullets).  This is really where the “harmonic” component comes into play – in that the various forces acting on the pellet combine ocasionally, by “chance”, in such a way as to reinforce one another to promote Ballistic Harmonic Spiral - the pellet spiral effect.

So the next time you weigh and select your pellets for a competition.  Use a magnifying glass and have a good look at the outside and towards the inside of each pellet.  It could give you an edge!

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Thanks

erifle Admin

admin@erifle.co.uk