Interior Ballistics

There are four types of ballistics, interior, exterior, terminal, and forensic. Today we will tackle interior ballistics. Watch my posts over the next few weeks to explore the other ballistic theories. These are very basic principles. Each one is an extensive and fascinating study in physics and math. Please do not let the math and physics scare you. I hope that you will continue to explore these fascinating theories. Interior ballistics begins when you pull the trigger on your gun and the sear releases the hammer. The freed hammer then drives forward. With the firing pin attached or free floating, it transfers its stored energy to the primer. The primer, which contains the only explosive chemical, now reacts. Gunpowder burns fast, it does not explode; more about that later. When the primer explodes, it releases heated gases into the cartridge chamber. These gases begin the process of burning the powder.

While the powder does not explode, it does burn very fast. This burning creates one of the most basic terms to grasp in ballistics: pressure. Pressure in effect creates force. Pressure creates force which acting per unit area overcomes inertia. The force overcomes the bullets state of rest or inertia. Mass has inertia and resist change in motion.

As the powder continues to burn, the pressure builds in the cartridge, creating more force. The walls of the chamber prevent the case from expanding outward. The bolt, or whatever seal is behind the case, secures the rear of the cartridge. The pressure and now the force reaches its maximum level and the only release is to act on the base of the bullet, the point of least resistance. In most cases this is somewhere around 10,000 Pounds per Square Inch (PSI).

Newton’s Law says, “An object continues in a state of rest, or in a state of motion at a constant velocity along a straight line, unless compelled to change that state by net force.” This law will come into play in most of our study of ballistics but for now, our bullet is in a state of rest. The bullet mass will resist the inertia or force. As the pressure increases, the force on the bullet becomes unbalanced.

Simply put, the net force at rest equals the mass (∑ F = m a) and we have no acceleration. Double the net force ((∑ F) and double the mass acceleration (m a). On the inverse, double the mass and cut the acceleration in half. Think of the cartridges you like best. The smaller ones, in grains, have more velocity than the big ones relativity speaking. Therefore, when the net force in the case exceeds the Inertia, mass of the bullet, it begins to accelerate. Acceleration is calculated by how quickly the velocity changes.

The primer, now struck and detonated, has ignited the powder, which begins to burn rapidly creating massive pressure that cannot escape. The pressure creating force far exceeds the ability of the bullet to remain in its state of inertia. The bullet overcomes the neck tension and exits the cartridge followed by massive gasses created within in the case. In less than one thousandth of a second maximum pressure is reached. The bullet is now at least one inch away from the brass case down the barrel.

You will see twisting inside the barrel, this is rifling. It makes the bullet spin and creates stability in flight. The raised areas are lands; the areas between the lands are groves.

As the bullet accelerates down the barrel, the lands will grab the bullet surface and begin the spinning of the projectile. Friction is now into play and energy is lost as the bullet moves down the barrel. Furthermore, pressure is decreasing as the bullet gets farther from the case as it moves down the barrel. Most cartridges with a velocity of 1,120 Feet per Second (fps) or faster, will make that velocity around one-third of the way down the barrel, which now exceeds the sound barrier.

Upon clearing the muzzle, gas pressure, released, equals about 30 percent of the original maximum pressure. This should clear up the argument of Newton’s third law with some shooters. Simply put, the law says, “for every action there is an equal and opposite reaction.” When you apply force to an object, move it, and make it go faster, you are doing work. That work is the kinetic energy of the object. If a bullet strikes an object or target with 2,000 ft-lbs and applying Newton’s law to the Kinetic energy of the gun, when you hold it would knock you backwards, hard.

When recoil happens, you are not experiencing the kinetic energy of the bullet but the net-energy utilized to create inertia, accelerate, and push the bullet down and out of the barrel. Thus, as previously stated, when the bullet exits the barrel only percent of that energy remains. Therefore, your recoil is equal to 30 percent of the original force used to accelerate the bullet down and out of the barrel. Furthermore, this 30 percent force disperses in numerous directions not just straight backwards. The bullet however, in motion in a direct vector retains its Kinetic energy until another force acts on it.

The muzzle blast you hear is not the powder exploding but a compressing of gasses escaping the barrel. The gasses devoid of oxygen burst when they encounter air.

When the pressure was builds in the cartridge, the case, especially brass, expands additionally sealing the chamber so gasses cannot release rearward. Pressure lost is energy lost. The final process is the cartridge case decompresses, especially with softer metals like brass. Thus, the used case removes easily. Cases made of steel can crack and jam in the chamber or cause cycling problems, as they are not as malleable.

All of the above has occurred in less than .003 seconds. Stay tuned, next time we will see what the bullet does as it travels from muzzle to target, exterior ballistics.

The Mission of Cheaper Than Dirt!'s blog, The Shooter's Log, is to provide information—not opinions—to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (2)

  1. Your section on recoil is incorrect. Recoil is a consequence of the law of conservation of momentum, which says that the net momentum must be zero. As you accelerate the bullet forward, it gains momentum (mass x velocity). The escaping gasses also have mass and velocity, contributing to the forward momentum. This forward momentum is opposed by the backward momentum, which is the recoil. A muzzle brake, which redirects escaping gasses to the side, decrease recoil because it takes that extra forward momentum out of the equation.

    The 30% net energy thing is crazy talk. Let’s look at a .223 fired from an AR. A 55gr bullet (3.564 grams) will exit at 3240 ft/s (987 m/s). That’s 1,736 joules of kinetic energy (1/2 mass x velocity squared). Its momentum, however, is only 3.517 kgm/s. That doesn’t account for the momentum of the escaping gas, so let’s multiply it by 5 just to be safe. That’s 17.585 kgm/s. That means a 7.5 lbs (3.4 kg) AR is moving backwards at 5.172 m/s. The rifle’s KE is thus 45.5 joules. That’s only 2.5% of the energy of the bullet.

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