As you likely already know, bullets spin during flight, due to the rifling of the barrel. But, you may have asked yourself: why do they need to spin? The answer has something to do with bullet shape.
In this article, I’ll show you the reason behind bullet spin with regard to bullet shape. Understanding this concept is useful to understand other bullet flight behavior that I’ll introduce in the future. Understanding how bullet shape affects its stability in flight is also useful in the process of selecting the right ammo, or the right bullet, for your rifle.
Bullets designed for long distances have all been engineered to fight against drag, so that they have a flatter and longer trajectory. For hunting and military purposes, bullets are also designed to maintain enough velocity, and therefore enough energy, to neutralize targets at long distances. These are the components of a standard long range bullet’s shape:
The cylindrical portion, which is the part that is forced into the barrel and is cut by the rifling.
The nose: Ogive shaped, and specifically designed to pierce through the air, fighting against drag.
The tail: In bullets designed for long range, the common shape is called boat-tail (tapered toward the end), and it is designed to reduce the turbulence behind the bullet and reduce drag.
On the bullet, we can identify two points:
The center of mass, that is, the point where the bullet balances its weight. When in fight, the center of mass is the only point of the bullet that actually follows the trajectory.
The center of pressure, that is, the point where the aerodynamics force, in this case the drag of the air flow (and a little bit of lift), act on the bullet.
As soon as it exits the barrel, the bullet starts its descending trajectory. As we have seen before, only the center of mass follows the trajectory. The tip of the bullet doesn’t follow the trajectory. In other words, the bullet axis is not pointed in the same direction as the axis of movement. In fact, the bullet longitudinal axis tends to remain pointed in the direction of the line of departure. Therefore, because of the bullet shape, the projectile will always fly at an angle, called angle of attack, relative to the trajectory. The result will be that the air flow will not push on the center of pressure from the front, but at an angle from below. Since the point of pressure is positioned in front of the center of mass, the air flow will force the bullet to rotate backward, pivoting on its center of mass.
In addition, due to its particular shape, the bullet has its center of mass behind its center of pressure. When the bullet is in flight, the center of pressure always positions itself in the direction of flight, that is, with the tail pointed forward, pivoting on the point of pressure.
If the bullet is not stabilized, these two forces acting together will cause the bullet to tumble. We can consider the distance between the center of mass and the center of pressure as a lever. The longer the “lever” is, the higher the destabilizing force it causes. Longer bullets, and especially bullet shaped with long ogives, have longer “levers” and thus are the most unstable.
In the next article, I’ll show you how the spinning motion, generated by the barrel’s rifling, keeps the bullet stable. Also I’ll talk about how to predict if a particular bullet can be stabilized by a particular twist rate, since this is important in the selection of the best bullet (or the best ammo) for our rifle.
AlessioBaldi drmorris9 I realize this was a year ago, but was just re-reading some of these and saw it. Yes, the center of lift shifts rearward starting in the transonic region and requires more and more down force from the tail. At the point you have inadequate down force to maintain your pitch, you enter “Mach Tuck” which is a uncontrolled pitch down that can very difficult to recover from. This is one reason aircraft have a MMO or Maximum Mach. For my aircraft it is .78 mach.
More good stuff….
drmorris9 I red about the issues of the CP shifting during supersonic jet flight, and the necessity to relocate the control surfaces. Something similar happens to the bullet during the transition from supersonic to subsonic, the passage through the so called “transonic region”. Very interesting stuff. I’ll dedicate an article to this 😉
When I started learning about long range shooting, I was pleased to find many of the terms and principals are similar to aviation (aerodynamics are aerodynamics regardless of what is flying through the air). Of course, in aviation we stabilize with control surfaces, and a CG in front of the center of lift, not with spin, so we never have to reference a Greenfield formula.
hartcreek I think I just answered to this question from you on a past article. The bullet maoght climb in relation to the line of the horizon, but it always drop relatively to the line of departure. If you shoot with the barrel parallel to the ground you won’t have any ascending part, because the bullet cant fight gravity… unless it is a Bullet With Butterfly Wings xD (sorry, I had this song of Smashing Pumpkins in mind LOL)
How is it that you can state that as soon as the bullet leave the barrel it statrs its decending trajectory when asllong as the bullet is exceeding gravity it is climbing. Bu deffinition a trajectory has to have to parts assention and desention.