There are irrefutable facts of life we can’t change or alter. Getting directly to point, there are distinct differences in ballistic performance between sub-sonic and super-sonic projectiles. Enough so, that each type requires a different ballistic shape and coefficient to describe a respective projectile’s flight. In other words, a sub-sonic, projectile-specific, BC. Unfortunately, as an industry, we’re still using firearm (typically super-sonic) BCs to describe sub-sonic pellet and slug flights. It appears this may change soon, due to efforts by folks like Bob Sterne. We can only hope.
Digressing for a moment. From here on it is assumed the subject covers airgun slugs, rather than pellets. After all, slugs dominate the sport of airgun hunting, which is this article’s main subject.
Part of any ballistic profile is the spin rate (RPM) of said projectile, which is imparted by the twist rate of the barrel. With respect firearms, the spin rates are much higher, and in fact can exceed 250,000 RPM in some varmint calibers. Most airgun projectiles, for example fall below 50,000 RPM. If you wish to calculate yours, here is the formula. Projectile RPM = MV X 720/Twist Rate (in inches).
Digressing again. Airgun projectiles are much softer than firearm projectiles. It should be obvious why. In any case, hardness is only mentioned to further set one of the subjects of this article.
Here is a the posed question, which sets the mood here on out—Why are airgun slugs deadlier to small game, than say a .22LR? In short, it is due to their malleable nature. That is to say, their softness (in comparison) allows them to expand more, thus producing a larger wound channel and subsequent tissue damage. On the other hand, the .22LR’s ‘slug’ is too hard to produce the same damage albeit with superior velocity and energy. One can argue a head shot one way or the other is equally deadly, and of course that would be so, if we were that lucky. We aren’t!
Fortunately, there is another factor which is seldom considered, and that is the spin rate of the projectile. In fact, projectile spin rate in an airgun can be more important than the energy expended. So here are the hypotheses.
As alluded to above, the softness of an airgun slug allows for greater expansion, even at the lower inherent velocities. Even more important is tumbling of the slug once it hits its target. Why this is so, requires a lot of math. The short answer is, the velocity decays much quicker than the spin rate does. In most cases (remember, not ALL cases!) The slower velocity with respect to the rotational speed (RPM), causes the slug to tumble (loose stability) once it hits its target.
Once more a digression. Projectiles tend to maintain ‘stabile equilibrium’ even when their ratio of spin rate vs. velocity falls below the point of stability, unless acted upon by external forces. In the case of small game, consider this the point of impact, also know as, ‘terminal ballistic function’.
The end result is more tissue damage than would otherwise be expected. This fact is easy to prove, by looking at high-speed (slo-mo) videos of strikes on ballistic gel. In comparison, pellets tend to remain stable regardless of velocity, as they’re partly stabilized by their (diabolo) skirt shape.
Enough said.
Digressing for a moment. From here on it is assumed the subject covers airgun slugs, rather than pellets. After all, slugs dominate the sport of airgun hunting, which is this article’s main subject.
Part of any ballistic profile is the spin rate (RPM) of said projectile, which is imparted by the twist rate of the barrel. With respect firearms, the spin rates are much higher, and in fact can exceed 250,000 RPM in some varmint calibers. Most airgun projectiles, for example fall below 50,000 RPM. If you wish to calculate yours, here is the formula. Projectile RPM = MV X 720/Twist Rate (in inches).
Digressing again. Airgun projectiles are much softer than firearm projectiles. It should be obvious why. In any case, hardness is only mentioned to further set one of the subjects of this article.
Here is a the posed question, which sets the mood here on out—Why are airgun slugs deadlier to small game, than say a .22LR? In short, it is due to their malleable nature. That is to say, their softness (in comparison) allows them to expand more, thus producing a larger wound channel and subsequent tissue damage. On the other hand, the .22LR’s ‘slug’ is too hard to produce the same damage albeit with superior velocity and energy. One can argue a head shot one way or the other is equally deadly, and of course that would be so, if we were that lucky. We aren’t!
Fortunately, there is another factor which is seldom considered, and that is the spin rate of the projectile. In fact, projectile spin rate in an airgun can be more important than the energy expended. So here are the hypotheses.
As alluded to above, the softness of an airgun slug allows for greater expansion, even at the lower inherent velocities. Even more important is tumbling of the slug once it hits its target. Why this is so, requires a lot of math. The short answer is, the velocity decays much quicker than the spin rate does. In most cases (remember, not ALL cases!) The slower velocity with respect to the rotational speed (RPM), causes the slug to tumble (loose stability) once it hits its target.
Once more a digression. Projectiles tend to maintain ‘stabile equilibrium’ even when their ratio of spin rate vs. velocity falls below the point of stability, unless acted upon by external forces. In the case of small game, consider this the point of impact, also know as, ‘terminal ballistic function’.
The end result is more tissue damage than would otherwise be expected. This fact is easy to prove, by looking at high-speed (slo-mo) videos of strikes on ballistic gel. In comparison, pellets tend to remain stable regardless of velocity, as they’re partly stabilized by their (diabolo) skirt shape.
Enough said.
