Internal Ballistics

phridum

Chicks Dig The Long Shot
Verified Sniper
Joined
Jan 4, 2008
Messages
62
Location
New Jersey
We have the terminal ballistics discussion so I figured I'd kick off the other two with a cut and paste from my physics presentation. It's basic and geared toward "hunters" to keep the sheep happy.

Please correct, add, or delete anything wrong, unclear, or TOO informative as per OPSEC concerns.

Science is very important in accurate shooting. Common misconception is that the precise placement of a round is a result of mysterious forces, super human talent, or even pure luck. Some even think that the extreme accuracy in sniping is impossible. In truth, consistent application of marksmanship and proper application of science, would allow a hunter to bring down even a running deer with confidence at 300 yards.

Many hunters rarely understand all that is involved when firing their rifles, despite their extreme confidence in themselves. A well educated and efficient hunter should understand all the ballistics and adjustments that goes into every shot.

Ballistics are divided into three distinct types: internal, external, and terminal.

Internal ballistics is the interior workings of a weapon and the functioning of its ammunition. It is what happens from the time the primer is ignited to the moment the bullet exits the muzzle. It is important for all of your bullets to be consistently the same so that every time you fire, the same things will happen and you can accurately predict the results.

When struck by the firing pin of the firearm, the anvil inside the primer crushes the mixture and the mix is ignited. This in turn, ignites the powder. The two chief primer designs in the world are Boxer and Berdan. America hasn’t accepted the use of Berdan mostly because of how difficult to recap it is.

The powder used in the round has evolved quite a bit over the years. Modern smokeless powder is of two types: Single based, and double based. Single based is composed of nitrocellulose. That’s a mixture of nitric acid and a cellulose, such as cotton. Double based powders are a mixture of nitrocellulose and nitroglycerine. Both are manufactured in three different forms: flake, ball (or spherical), and tubular.

These modern powders are progressive in their combustion rate, meaning that their rate of combustion is controlled. Some of them burn fast, some of them burn slow. The burn rate is controlled by the size of the powder granules, their shape and chemical retardants added as a coating to the powder. The tubular, also known as extruded, and flake powders are designed with a hole through the center of each granule. This allows each flake to maintain a constant surface area as it burns which permits a consistent rise in pressure in the barrel.

Ideally, peak pressure would be attained just prior to the bullet exiting the muzzle. Hence the need for various burning rates. Theoretically, the shorter the barrel, or lighter the bullet, the faster rate of powder burn needed for best efficiency. The ideal barrel length will be just long enough to allow the peak pressure to form. As the pressure rises, it accelerates the bullet to higher velocities. For example, for a modern centerfire .308 Winchester, the length of barrel needed to attain peak pressure, complete combustion, and maximum muzzle velocity is between 30 and 36 inches. However, such a length is impractical and unwieldy for common use. Therefore, some potential velocity is sacrificed as practical compromise for ease of handling and efficiency of design. This is why there is the noticeable muzzle flash of the remaining powder combusting after the bullet exits the muzzle. The resulting sound and concussion is caused, not by the powder, but is a little “sonic boom” of expanding gas escaping from the muzzle at supersonic speeds.

The generation of the 50-55,000 pounds per square inch of pressure within the rifle is the driving force of internal ballistics. It has numerous effects upon the cartridge case, the rifle and the bullet, all which affect the inherent accuracy of that particular rifle and cartridge combination. Every time a shot is fired, the various parts of a rifle, cartridge case, and the bullet all move and interact with each other. The more nearly these parts move and interact exactly the same way for each shot, the more accurate that rifle and cartridge combination will be.

The cartridge has four functions: a container for the powder, bullet and primer, to cause pressure to build equally in all directions, to provide proper headspace, and to provide uniformity of position within the chamber.

As the powder ignites, the flash temperature in the chamber of the rifle exceeds 3500 degrees Fahrenheit. Cartridge cases are made of brass because it expands easily and uniformly. The expanded case forms as gas tight seal causing the path of least resistance to be through the bullet and down the barrel. As the pressure dissipates, the brass shrinks quickly enough to be easily extracted from the chamber.

The third purpose for the case, providing proper headspace, is basically the exact measure of the bolt face where the head of the round sits, and the point that prevents the cartridge from going farther down the barrel.

The uniformity of position within the chamber applies to the base and neck of the cartridge. If the case head doesn’t sit square to the bolt face, the pressure generated will torque the case within the chamber, causing the bullet not to enter the bore concentrically. As the bullet sits in the rifle before being fired, it touches nothing but the case. As it leaves the case, it needs to contact the lands, which is the raised area of the rifling, in perfect alignment. Otherwise it will cause a slight deformation of the bullet and upon exit, will not spin true about its longitudinal axis, thus throwing off it trajectory unpredictably. On the more advanced level, this throws off the barrel harmonics, which can compound the disruption of trajectory as well.

In firing, four different vibrations are imposed simultaneously upon the barrel. Initial vibration is from the rifle recoiling to the rear in accordance with Newton’s Law that for every action there is an equal but opposite reaction. The direction of the recoil force is directly opposite to the movement of the bullet down the barrel. The impact of the bullet into the rifling coupled with the impact of the stock against the shoulder causes the rifle to literally buckle in the middle. This occurs at the junction of the barrel and the receiver, and springs back. The result is vertical vibration of the barrel much like a whip.

There is sharp sudden impact when the larger diameter bullet connects with the smaller diameter lands in the rifling before it is forced down the barrel. This creates longitudinal vibrations within the barrel.

Torsional, which helps to be thought of as rotational or spinning, vibration is caused as the bullet is forced down the bore against the turning of the rifling.

The side to side lateral vibrations can be caused by any flaws in the bedding of the receiver to the stock, inconsistencies of the shooters grip or position, abnormalities in the case head or cocking lug dispersion.

All these vibrations will reverberate lengthwise within the barrel several times before the bullet leaves the muzzle. We call this resultant motion, barrel whip. The barrel whip creates a sine wave pattern. The high points of the pattern are called anti-nodes. This is where the barrel movement is at its greatest. The low points in the waves are called nodes. The barrel whip factor would be inconsequential if the muzzle of the barrel were always a node since the muzzle would be at the same point in space every time the bullet exited. However, due to the mechanics of the sine wave, it is impossible to make it so. The muzzle is always an anti-node and will always be at the maximum vibrational motion. Therefore, in order to be accurate, the barrel must always whip with exactly the same motion from shot to shot in order for the muzzle to be always at the same point upon exit of the bullet. Whip can be lessened and accuracy increased with high quality, heavy barrels. This is also the reason for free-floating barrels and the importance of not letting anything touch the barrel when supporting the rifle.

The last factor of internal ballistics concerns the bullet and its movement through the barrel. The greatest concern of the effect on accuracy is the rate of twist of the rifling. For example, 1 in 12 rifling makes one complete rotation of the bullet every 12 inches of barrel and is termed a “12 inch twist”. The twist controls how fast the bullet spins. Spin stabilizes the bullet in flight so it will fly nose first. The longer and heavier the bullet, the faster the twist necessary to stabilize it.

The velocity also affects the proper choice of twist. Increasing velocity will increase the revolutions per minute of the bullet. Optimum accuracy is obtained by using one bullet weight and barreling with the best twist for the caliber, weight, and velocity combination. Typically, rifle calibers are between 150,000 and 200,000 RPM’s.

The critical feature of the bullet while in the barrel is its base. This must form a perfect seal to contain the mounting pressure behind it. If the base is not square, at the instant the bullet exits the muzzle an uneven push is exerted by the pressure. The resulting axial error can result in a group size increase of one minute of angle even if it’s only .001 of an inch. Because it’s technologically difficult to manufacture perfectly square bullet bases, the boat tail design is more efficient.
 
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