So this morning I started thinking about the tuning screw at the back of my Kalibr Cricket (5.5mm), posted a question on a thread and because of my profession was asked to maybe write something about the power of air rifles and the transition between sub- and supersonic flight of pellets.
Which is actually very interesting as I started thinking about it in shooting terms and not an aeroplane!

So here goes I know a lot of you experienced guys may know this but if there are a few that will learn something I will feel quite chuffed Lol. If there are some spelling issues I apologize as I am Afrikaans!

I think we all want the most power out of our air rifles, but is more power better and when is enough?
It’s easy to get an air rifle to shoot at supersonic speeds, either buying it like that or getting the rifle tuned. But what happens when shooting a pellet at speeds greater than the speed of sound vs a firearm which only shoots at supersonic speeds are two very different worlds!
Breaking it down:

  1. What is the Sound Barrier and how does it work?
  2. The difference between Pellets and Firearm bullets



What is the Sound Barrier and how does it work?
The sound barrier is where an object ie. Bullet, pellet, aeroplane moves faster than the speed of sound!
Now, there is no real fixed speed where the speed of sound lies! This is because of the Temperature, Pressure, Altitude and Humidity is not the same on any given day!
In aviation we have an ISA ( International Standard Atmosphere) environment with obviously a lot of unrelated info to air rifles, but ISA says that:

In dry air, at 20 degrees Celsius (68F) and at sea level that the sound barrier will be reached at 343m/s or 1125ft/s!


Also speeds beyond that of the of sound are not referred to as km/h or m/hr but rather a Mach number ie. Mach 1.32 ( 1.32 times the speed of sound)


For explanatory purposes of the effect of Temperature, Altitude etc. on the speed of sound see the table below:
Effect of temperature on properties of air
Temperature T in C Speed of sound c in ms−1 Density of air ρ in kgm−3 Acoustic impedance Z in Nsm−3
+35 351.88 1.1455 403.2
+30 349.02 1.1644 406.5
+25 346.13 1.1839 409.4
+20 343.21 1.2041 413.3
+15 340.27 1.2250 416.9
+10 337.31 1.2466 420.5
+5 334.32 1.2690 424.3
0 331.30 1.2922 428.0
−5 328.25 1.3163 432.1
−10 325.18 1.3413 436.1
−15 322.07 1.3673 440.3
−20 318.94 1.3943 444.6
−25 315.77 1.4224 449.1
Given normal atmospheric conditions, the temperature, and thus speed of sound, varies with altitude:
Altitude Temperature ms−1 kmh−1 mph knots
Sea level 15 C (59 F) 340 1225 761 661
11,000 m−20,000 m
(Cruising altitude of commercial jets,
and first supersonic flight)
−57 C (−70 F) 295 1062 660 573
29,000 m (Flight of X-43A) −48 C (−53 F) 301 1083 673 585
The difference between Pellets and Bullets:


The Pellets in the top row are 5.5mm cal and the bottom row 4.5mm

Now, a pellet consists of the “head” either flat, dome, hollow or pointed. Then the “neck” which is below the head thinning the pellet and then the “skirt” ,the bottom part of the pellet.
Air rifles in the chamber shoot at 50-120 atmospheres of pressure where firearms does in the 1000’s of atmospheres!
As the pellet is shot through the barrel the skirt takes on the grooves as this is how spinning is achieved.
See example:


The lines on the skirt are clearly visible in the picture.
Due to a pellet being made out of lead which is fairly soft, at subsonic speed as you know are very accurate and consistent, but when shooting a pellet at speeds approaching the speed of sound the more unstable they become! This is mainly due to the Aerodynamic build of a pellet against a bullet and the lead starting to de-form or break up in air making the pellet tumble when shooting beyond the speed of sound!
A pellet being “Diabolo” shaped and non-spherical (bullet) combined with the soft lead can’t handle the pressures at supersonic speed and thus the de-forming and tumbling.


As in the figure above

  1. Subsonic
  2. Mach 1 ( the speed of sound)
  3. Breaking the speed of sound
  4. Shock wave

Fig 1: Subsonic, great for pellets, they are designed to fly there!
Fig 2 : This is AT the speed of sound and the pressure line as seen above is still in front of the object but this is where a pellet will become unstable.
Fig 3: What we have now is the pellet being shot at a speed greater than the sound barrier, clearly visible above is the pressure line being curved into a V-shape and thus forcing a lot more pressure on the pellet, this then where skirt first as it is not solid,starts to crumble getting more unstable and finally tumbling!
When a pellet holds together through the sound barrier and starts to decelerate from super- to subsonic even if not breaking up,as it passes through the pressure line it will again become unstable and tumble!
A lot of tumbling? A bullet is designed to fly at supersonic speed being much longer, heavier and aerodynamically efficient, a pellet is not.
Yes, a lot of air rifles do shoot supersonic. Both PCP’s and big Springers! My very first PCP was an Hatsan AT-44 4.5mm shooting at 1143 ft/s on the chrony and it was deadly out to 50m.
Ever wondered why your perfectly sighted in rifle throws a flyer too often?
Go shoot that airgun at greater distance and see what the POI does, have a close look at the impact on the paper, the tumbling causes the pellet to hit sideways and make a hole looking like a keyhole.
More Power? Nothing wrong with that, I am going to turn up the Cricket but just won’t be able to shoot the 16gr pellets for example but now a 18 or 23gr pellet, just to keep the pellet from breaching the sound barrier. Keeping it stable, consistent, accurate and of course the added benefit of having much more kinetic energy down the range!
And that my whole 2c worth! Hoping somewhere it helped Somebody! J

Speed of sound in air calculator: http://hyperphysics.phy-astr.gsu.edu...nd/souspe.html

Regards
Hein