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Author Topic: Factors in bullet stabilization  (Read 222 times)

Offline Dutch-Hunter

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Factors in bullet stabilization
« on: October 21, 2018, 10:24:13 AM »
My usual disclaimer... Remember you ask... I got my stuff together... and wrote this answer... for what its worth

The definition of projectile stability is, maintaining the nose of the projectile pointed in the direction of travel. This is achieved by the projectiles propulsion and rotation if needed. A rocket has tail fins to correct nose deviations. As the nose pitches in one direction the tail fin counter pressure is increased which corrects the nose position. Does a bullet have tail fins? Answer, sort of! The scares in the jacket by the rifling of the barrel are mini fins. Do these mini fins help in stability? Answer, well sort of! Because of the physics of spinning a propelled object creates pitch and yawn flight characteristics. Bullets are no exception. There are several physical forces that affect the stability of a bullet.

The factors of bullet stability are mass weight, distance of travel and velocity. There are two major physics stability factors to consider. The first is Gyroscopic and Dynamic is the second.

Gyroscopic is the easiest to understand and explain and is abbreviated in calculations as Sg. Sg increases with rate of rotation. It also goes up as velocity decreases. Sg also changes with bullet geometry. It goes down with long thin projectiles and up with short fat ones.

Dynamic is much harder to explain and determine and is abbreviated as Sd in calculations. Sd is dependent on bullets geometry or features and design. This is different for each and every bullet which compounds calculating the affect. Simply put if the bullet can be gyroscopically stabilized it can be dynamically stabilized too.

The longer a bullet is for its’ diameter, the faster the twist has to be to gyroscopically stabilize it. In the case of the .223 with a 1-8″ twist, this was designed to stabilize 80 gr. bullets in this diameter. In truth the opposite is true. A 1-8″ will spin a 55 gr. faster than what is required in order to stabilize that length of bullet. This rule of thumb is based on the fact that lighter bullets generally travel faster because of the mass weight differential. If you have a bullet with good concentricity in its jacket, over spinning it will not hurt its’ accuracy potential. However there comes a point at which overspinning a bullet may be structurally unable to withstand the rotational forces generated. In this scenario the bullet self-destructs in flight.

Another rule is if a bullet leaves the muzzle in an unstable manner it will remain unstable in flight. For this reason we’ll now look down range for some another interesting phenomenon. Most centerfire rifle bullets come out of the muzzle at a velocity rate of 2300 to 4700 feet per second. Or in supersonic terms a Mach speed of 2.06 to 4.21 at sea level. Now that’s movin’. As the bullet moves leaves the muzzle it begins to slow down. Velocity drops at a rapid rate, spin rate slows much slower.

When the bullet decelerates to transonic speed, which is when the bullet slows to about 1340 feet per second. It is getting close to the speed of sound. For bullets decelerating to the sound barrier has a destabilizing effect. The center of pressure moves forward, and the over-turning moment on the bullet gets greater. To remain stable in flight the bullet is going to have enough gyroscopic stability to overcome the increasing dynamic instability that’s experienced at transonic speed? Some bullets do this better than others. Typically bullets that are shorter and have shallow boat-tail angles will track better through the transonic range. On the contrary, bullets that are longer can experience a greater range of pitching and yawing in the transonic range that will depress their ballistic coefficients at that speed to greater or lesser extents depending on the exact conditions of the day. That makes it very hard to predict your trajectory for bullets like that through that speed range. Thus increases the effectiveness at extended ranges. These extended ranges are typically over 500 yards.

When you look at transonic effects on stability, you’re looking at reasons to maybe have a super-fast twist rate to stabilize your bullets, because you’re actually getting better performance therefore you’re getting less drag and more BC from your bullets if they are spinning with a more rigid axis through the transonic flight range because they’ll be experiencing less pitching and yawing in their flight. However as stated earlier over-spinning in not without its risks.

Consider an interesting semi-related phenomenon, add a higher altitude into the equation and look at the difference in the aforementioned muzzle velocity speeds. At 10,000 feet above sea level sound actually moves slower because the air is less dense. Using the same muzzle velocities as before the Mach speed range translates to 2.34 to 4.78.

How altitude affects the flight of a bullet. The simply put, at higher altitudes, the air is thinner, so there is less drag on the bullet. This means that the amount of bullet drop is less at any given flight distance from the muzzle. Since the force of gravity is essentially constant on the earth’s surface. The bullet’s downward acceleration doesn’t change, but a bullet launched at a higher altitude is able to fly slightly farther for every increment of downward movement. Effectively, the bullet behaves as if it has a higher ballistic coefficient.

In basic terms, as your altitude increases, the density of the air the bullet must travel through decreases, thereby reducing the drag on the bullet. Generally, the higher the altitude, the less the bullet will drop.

Here’s a thought challenge for you, is it easier to stabilize a bullet at higher altitude? Think on it for a while and we’ll get back to it later.
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Offline BoBallistic

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Re: Factors in bullet stabilization
« Reply #1 on: October 21, 2018, 11:43:38 AM »
Dutch - Thank You for a good explanation about bullet Stability and the difference in Sg and Sd.......when we say every thing matters, we mean everything....Sierra Points this changing dynamic in the BC as the bullet slows down the BC along with the Sg and Sd changes and Dutch states the Gyroscopic of a bullet known as the internal ballistics is easy to calculate and determine...what is the hardest to solve and determine is after the bullet leave the barrel, called external ballistics or dynamic....there are so many factors to considered, in the actual world....Another physics class by Dutch...

Would love to be a fly on the wall when Sierra, Hornady, Nosler, Remington, Barnes, Speer, Swift, Lapua (wonder what language they speak in their meetings) and listen to them explain the internal and external ballistics of a new bullet....Dutch could hold a class with these guys!!!

But, sometimes after doing all your homework, and believe me when I say there is a lot of homework to do!!, and you have worked up a great load and you are ready to go hunting, maybe just maybe a "gut" feeling is what you must determine and squeeze the trigger and hoping your bullet finds your mark in the woods....

Thanks again Dutch for taking time out of your schedule to answer a good can tell it much better than most of us....that is why you are proud to be a gun geek!!

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Offline Madgomer

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Re: Factors in bullet stabilization
« Reply #2 on: October 21, 2018, 10:08:34 PM »
I'm casting my vote for "easier to stabilize at higher altitude"
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