For a long time it was thought that the only force a bullet had to overcome was the force of gravity. Air was thought to have such a small influence on the path of a bullet that it was not even involved with determining early trajectories. Later, Benjamin Robins, using his ballistic pendulum, showed that the air had an influence 85 times greater than that of gravity on the flight of a bullet. Since that time great effort and energy have been involved in trying to minimize the effect air has on a bullets flight path. From the .75 caliber monster fired by Robins to the sleek .308 shot by many hunters today, drag has always posed a problem in accurately firing a rifle.

          There are many factors involved in understanding how air effects the trajectory of a bullet; some of these include air pressure (altitude), humidity, and temperature. All of these things effect the path of a bullet in flight, and must be considered to accurately fire a rifle. Other things that influence a bullets trajectory are the bullets shape, its spin ratio, and its velocity. This discussion will mainly focus on the laws of motion and how different forces (air pressure, humidity, temp.) influence the path of a bullet.

          Sir Isaac Newton (1642-1727) was perhaps the greatest scientist of all time. One of his greatest contributions to science, and the field of ballistics, was his formulation of the three laws of motion. The three laws of motion describe how things act with respect to gravity and forces which act upon them. Newton?s first law of motion states:

A body remains at rest, or if in motion it remains in uniform motion with constant speed in a straight line, unless it is acted on by an unbalanced external force.

The acceleration produced by an unbalanced force acting on a body is proportional to the magnitude of the net force, in the same direction as the force, and inversely proportional to the mass of the body.

Whenever one body exerts a force upon a second body, the second body exerts a force upon the first body; these forces are equal in magnitude, and oppositely directed.
 
 

          First of all, the bullet in the chamber is ?at rest? and must be acted upon to accelerate out of the barrel. This initial force is, of course, the burning of the gunpowder, which causes the subsequent expansion of gas forcing the bullet out of the cartridge, down the barrel, and toward the target. Interior ballistics is the study of this process; from primer ignition to the bullets exit from the barrel. The bullets flight path out of the rifle is influenced greatly by many variables inside the rifle, all of which play a part in the exterior and terminal ballistics of the bullet. The expansion of the gas inside the cartridge is what drives the bullet to its maximum velocity; the gas taking the path of least resistance in the process of diffusion causes this velocity increase. With regards to Newton?s first law the process can be summarized as the bullet being the object at rest, and the expanding gas as the unbalanced external force. As the unbalanced external force acts on the bullet it pushes it out of the barrel. Another external force acts on the bullet as it is pushed down the barrel; this second force is the rifling, which produces the spin on the bullet and causes a gyroscopic stabilization of the projectile. Though there are other forces which act upon the bullet while in the rifle, consideration of these two very important variables will suffice, as they seem to be the most influential factors with regards to exterior ballistics.

          Now the bullet is in motion and, according to Newton?s first law, the bullet would stay in uniform motion unless acted upon by an external unbalanced force. So what are some of these external unbalanced forces? The most obvious is gravity; the other is air resistance, or drag. These two forces are what cause a bullet to deviate from its uniform path and they are the forces that constitute the study of exterior ballistics. Gravity and resistance to air are the forces that determine what requirements a ballistician must incorporate into a bullet to try to achieve the best flight possible. These two forces can be partially overcome by understanding Newton?s second and third laws of motion.

          Gravity is a constant; it is a force acting on a bullet pulling it towards the center of the earth at 32.0 feet per second squared. For example, after one second of flight a bullet has fallen 16 ft according to the constant force of gravity. It is an unbalanced force in that it only acts in one direction, down. The only partial solution to overcoming the effects of gravity is to increase velocity, because the faster the bullet goes toward a target the less time it has to be effected by gravity. Other than increased velocity there is very little that can be done to minimize the effects gravity has on a bullet.

          The bullets resistance to air is another force that acts to effect the flight of the projectile. Its influence is much greater than the influence of gravity, but the influence of air on a projectile can be reduced to a much greater extent than the effect of gravity. The general shape and design of our bullet are such as to optimally reduce the drag created from the bullet cutting through the air. Air can be considered an unbalanced force in that it acts on different parts of a bullet to differing degrees, which unless properly adjusted for can cause problems with stabilization. Most obviously the drag caused by air acts on the front of the bullet as the bullet pushes the air out its way. It also acts on the back of the bullet when a vacuum is created from the bullet flying through the air. We can apply Newton?s second law by saying a bullet is propelled toward a target by overcoming all the forces that are acting on it, and the more the effects of these forces can be minimized, the more optimal the flight of the bullet will be.

          Drag can be understood in very simple terms by applying Newton?s third law of motion. For every action there is an equal and opposite reaction, so for every particle of air the bullet has to move out of its way that particle of air exerts a force equal to its own mass and oppositely directed to the path of the bullet. This is the concept of drag. So it makes sense that at greater air pressure (more particle per space of air) there will be more drag on the bullet. This same concept applies to temperature (colder air is more dense), and to humidity (greater humidity equals lesser density). So if we can minimize the drag imparted by the bullet cutting through the air, the better it will fly. The problem with this is being able to minimize drag at high velocities, keeping the lethal potential of the bullet, and having the bullet hold together under the pressure of all the forces acting upon it.

          Additional problems with drag are introduced as a bullet approaches and surpasses the speed of sound. While at subsonic speeds a bullet has minimal amounts of drag acting on it and it is relatively easy to keep a stable accurate bullet at these low velocities. At transonic speeds, which begin at about 800 fps, a shock wave begins to form around the bullet as the air, which is being pushed aside, must move at about the speed of sound, (1120fps), to get out of the bullets way. The transonic speed that all bullets pass through when accelerating and decelerating is the most unstable speed. It is an inherently unstable speed because the bullet is beginning to form or lose a shockwave. What this means is that the air around the bullet cannot move out of the way fast enough, and so begins to pile up in front of the bullet. This is what makes aerodynamics very important in streamlining a bullet, and it is what makes it so difficult to stabilize a bullet. This transonic stage of a bullets flight is extremely turbulent and so it presents the largest problem for a ballistician to overcome in creating an accurate bullet at long distances. But, stabilizing a bullet with spin and increasing its aerodynamic properties can overcome these problems. At transonic speeds a shockwave is beginning to form, as speed increases to supersonic this shock wave grows. This means increased drag from the shock wave requiring energy to be pushed along with the bullet. However, supersonic speeds are much more stable than transonic speeds, but the trick is getting a bullet through the transonic stage and retaining its stability. The bullet must provide the energy to push both itself and the shock waves produced in the transonic and supersonic speeds through the air. This increased energy required to propel both the bullet and the shock wave through the air is another force which must be considered when understanding the effects of drag on a bullet.

          At high velocities, under all these forces, many bullets will simply fall apart, or vaporize. So in order to allow a bullet to hold together there were limitations placed on what a bullet could do, and more than anything else velocity was sacrificed. At Lost River Ballistic Technologies we don?t believe in sacrificing velocity, because more velocity means more energy transferred to the target, which means more lethality. So we decided to make bullet?s that can hold up to the intense forces put on them at high velocities. We have also maximized our bullets resistance to the effects of drag by giving the best shape and balance so as to streamline the bullet in flight. With this new design we have decreased the effects of drag and increased velocity. By increasing velocity we decrease the effects of gravity with respect to distance traveled by the bullet and increase lethality. By finding a way to reduce the effects of gravity and air drag while increasing lethality we have come to offer the best hunting bullet ever.

Matthew Mosdell

At Lost River Ballistics.