Supersonic, Transonic and Subsonic Ballistics
Any bullet fired from a high powered rifle will go through the Supersonic, Transonic and Subsonic phase when fired over extreme range.
What this basically means is that the bullet will be moving faster than the speed of sound, then move through the sound barrier, and then move slower than the speed of sound until the bullet, in theory, comes to a complete stop.
Let me state this up front, I am not a ballistics’ engineer, so this is the very basics of what actually happens, but it gives a very good idea of what actually happens to a bullet, and how much design and engineering work is done to find the optimal bullet shape, weight and profile.
What is the speed of sound?
First of all let’s define what the speed of sound is. Contrary to what a lot of people believe the speed of sound is not a fixed value, in dry air at 20 °C, the speed of sound is 343.2 metres per second (1,126 ft/s). This is 1,236 kilometres per hour, or a kilometre in 2.914 seconds, or a mile in 4.689 seconds.
At sea level a generally accepted value is 340.29 m/s, and this value decreases as the altitude increases.
Note the two variables, dry air and temperature! because gas molecules move more slowly at colder temperatures this slows the speed of sound, sound moves faster through warmer air.
Thus the speed required to break the sound barrier decreases at higher elevations, where the temperature is colder, and the same applies to dryer air, which is less dense.
The speed required to break the sound barrier is known as Mach 1, thus Mach 2 is twice the speed of sound, Mach 0.5 is half the speed of sound.
An interesting fact, the fastest manned flight was done by NASA’S X-43A aircraft on November 16 2004, the aircraft flew at Mach 9.6, almost 7000 mph, which is 11265 km/h.
That is fast!
But why is this relevant to ballistics?
When a high powered rifle bullet is fired it’s initial flight is supersonic, that is it is moving faster than the speed of sound. At that speed the air resistance is equal to hitting a brick wall, and the bullet has to move through this, so a pressure wave is created in front, and on the tip of the bullet. If you read my article on the RPM of a bullet, you will see that the bullet is spinning on a axis, which runs through the centerline of the bullet. The bullet also has a center of gravity, which is in roughly in the centre of the bullet.
What a lot of people don’t know is that the bullet also has a center of pressure, which is in front of the center of gravity when the bullet is flying at supersonic speeds.
When the bullet approaches the transonic range, that is it is moving through the speed of sound the center of pressure starts to move backwards as it slows, and moves towards the center of gravity which remains static.
This happens because the pressure wave is no longer in front of the bullet, but moves down the bullet as the speed slows.
Transonic speed for a bullet is generally accepted to be between Mach 1.2 and Mach 0.8.
So what, you may ask?
As the center of pressure approaches the center of gravity funny things start to happen, the bullet starts to wobble, and if the bullet design and weight is not correct, the bullet will start to tumble when the two centers equal each other, or swop positions.
You can imagine what this does for accuracy!
I first noticed this when I was doing my military service, at 800m sometimes a 5.56 bullet would hit the target side on, and I was always under the impression that the bullet had hit the ground first and bounced through the target, clearly not the case! What was actually happening— the bullet was tumbling nose over tail when it hit the target.
When a bullet is travelling at subsonic speeds the pressure wave is basically at the tail of the bullet, and the centre of pressure is at the tail of the bullet, and no longer has an influence on the bullet flight
.
This is the problem that bullet designers and manufacturers face, how do you stabilise a bullet moving through the transonic phase?
There is a lot of work done in this area, and I am not even going to attempt to explain this.
Faster is not always Better!
So— I started to do a bit of research on velocity of bullets and the relationship to accuracy, and came to some surprising conclusions.
Like a lot of us out there I am aware that there is a optimum speed, or velocity, that is required for bullet accuracy, but I was not aware of the relationship between spin rate, velocity and bullet stability.
First of all, do you know how fast a bullet is actually spinning? A standard .30 caliber round fired from a standard .30 caliber rifle is spinning at around 200 000 RPM! That is insanely fast, you can imagine the forces being exerted on the bullet.
To find out how fast the bullet is actually spinning is pretty simple, this is the formula used: –
RPM = ((12/T) x V) x 60
Where T = Twist Rate, and V = Muzzle Velocity.
So, using this formula I worked out the spin rate on my .303.
RPM = ((12/10) x 2460) x 60 = 177 120 RPM
(T =1:10, and the muzzle velocity for Federal Power Shock ammo is 2460 fps.)
we all know that the faster the bullet is, the flatter it shoots, and thus the less bullet drop there is, but if we increase the bullet velocity by too much we can create all kinds of problems.
- The faster the bullet, the higher the barrel wear. I have heard that a barrel life of about 1000 rounds can be expected from a 7mm Magnum or a 300 Magnum firing high velocity rounds.
- There is a very real danger that the spin can cause the bullet to disintegrate, from the formula you can see how much of an influence the velocity has on the spin rate.
- Thirdly, we all know that the spin imparted on the bullet increases it’s stability, but a bullet spinning to fast will have an adverse effect on it’s accuracy over distance— here’s how. The spin on a bullet creates gyroscopic stability, that is the bullet is spinning on the center of its axis’. Now when it comes to shooting at distances of 500m or more, this extra speed of spin keeps the bullet in a nose up position, causing the bullet to drop onto the target in a belly down position, rather like a belly flop into a swimming pool!
Which is Better: – .17 HMR or .22 WMR?
Friends of mine know I have had both calibers, and asked me, in my opinion which one is better?
I have done a lot of reading on the subject, I have my own personal preference which I will talk about later, the decision I made was long before I started to do research on the subject and I still think I am right, but everyone has their own opinion.
First of all, a little history on the two cartridges: –
The .22WMR
The .22 WMR, commonly known as the .22 Magnum, has been around since 1959, and was the most powerful rimfire cartridge available until the advent of the .17 HMR. In terms of muzzle energy, the .22 Magnum is still the king.
The original .22 Magnum came out with a bullet weight of 40 grains, which was the same as the .22 LR bullet, but the bullet was fully jacketed, and came in two formats, the jacketed hollow point (JHP), of full metal jacket (FMJ). The original muzzle velocity (MV) was 2000 fps.
Since then the major manufacturers have reduced the muzzle velocity to 1910 fps for the 40 grain bullet, but RWS still makes a 40 grain cartridge at a MV of 2020 fps
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There are of course lots of different choices out there now, to name a few: –
- CCI Maxi –Mag and Federal V-Shock, 30 grain at 2200 fps
- Remington 33 grain at 2000 fps
- Winchester 33 grain at 2120 fps
- Federal game shock 50 grain at 1652 fps
- Winchester 45 grain at 1550 fps
The .17 HMR
The .17 HMR came as a result of a joint collaboration between Hornady, Marlin and Ruger, and was first introduced in 2002. It was an immediate hit, and for the first couple of years demand for the ammunition far outstripped the demand. Hornady was the first company to supply the ammunition, but since then all the major manufacturers have stepped up to the plate.
The .17 HMR is basically a .22WMR cartridge necked down to accept the .17 bullet, and the original design fired a specially designed bullet, 17 grain V-Max, at a muzzle velocity of 2550 fps. The bullet is a polymer tipped, spire point boat tail bullet, and is designed to fragment in small animals and to disintegrate if it hits a hard surface. This bullet became known as the Varmint Grenade.
In 2004 a new 20 grain bullet was introduced, designed to be less destructive, for better penetration in small game and predator’s.
There are also lots of different choices out there: –
- Remington 17 grain gold tip at 2550 fps
- Federal V-Max at 2550 fps
- CCI 20 grain Game Point at 2375 fps
One thing to note, Hornady prides itself in loading their ammunition to the greatest accuracy possible, designed to deliver 1 or better MOA at 100 yards.
So how do they compare?
Velocity
Winning hands down is the .17HMR. At 100 yards the .17 HMR is moving at an average speed which is 550 fps faster than the .22 Magnum.
Energy
Velocity and bullet weight are the two important factors in calculating kinetic energy which powers bullet expansion and penetration, and thus killing power.
At 100 yards the 40 grain .22 Magnum is the most powerful round, but loses its energy very quickly after that.
Trajectory
Velocity has a huge effect on bullet trajectory, the faster the bullet the flatter it will fly. This is also true for the ballistic coefficient. Here the better design of the bullet in the .17 HMR comes into play, as well as its superior velocity.
Again the winner is the .17 HMR.
Sectional Density
This is the bullet weight divided by its square of the diameter. A bullet with a greater sectional density will penetrate deeper.
Here the .22 Magnum, with its heavier bullet wins, again at 100 yards. Any distance over that the .22 Magnum has lost most of its velocity.
Accuracy
The winner hands down is the .17 HMR, if the wind is not blowing. Because the bullet is so light and small the wind has a huge influence.
Conclusion
Both calibers are useful for small game and varmint hunting, which is what they are designed for. The .22 Magnum, with its heavier bullet weight and sectional density is the one I would prefer for hunting in distances up to 100 yards, but nothing much more than that.
The .17HMR does not have the “knock down power” that the .22 Magnum has out to 100 yards, but it does out class at ranges beyond that, and is far more accurate.
The bottom line is it is a matter of personal preference. I sold my .17 HMR and kept my .22 Magnum because where I live the wind is always blowing, and I found the .22 to be more accurate.
Impressum
Tag der Veröffentlichung: 26.04.2016
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