The same defect aft of the center of gravity (that is, at the base) will cause the two effects to reinforce each other. It turns out that a defect forward of the bullet's center of gravity (that is, at the nose) will cause this cancelation. Cover Your BasesĪn interesting side note is that aerodynamic jump can partially cancel out lateral throwoff. The faster the barrel twist, the more deviation in the trajectory. Defects in bullet balance or in-bore tipping will cause lateral throwoff and aerodynamic jump, which will cause the bullet to fly off in an unwanted and unpredictable direction. Spin is required to stabilize bullets in flight. Again, the deflection will be in a random direction determined by exactly how the bullet was positioned when it left the muzzle. In any case, the faster the barrel twist, the more aerodynamic jump you will have. As we'll see in a moment, wind can also cause aerodynamic jump. Like lateral throwoff, aerodynamic jump can also caused by in-bore tipping.

Similarly, when dynamically unbalanced bullet (even if it is statically balanced with the center of gravity perfectly centered on the bore) leaves the muzzle, it will do so with an initial yaw that causes it to fly off in the wrong direction. In other words, faster twists are less accurate with imperfect (that is, real) bullets. The faster the barrel twist, the more lateral throwoff you will have. All that is required is for the center of gravity of the bullet to be off the bore's centerline. Note that a perfect bullet, if it is allowed to tip in the bore, will also fall prey to lateral throwoff. The exact direction depends on what direction the heavy side of the bullet happens to be facing when it leaves the muzzle. When the bullet leaves the muzzle, it will fly off in a direction tangent to that helical path. When a statically imbalanced bullet travels down the bore, the off-center center of gravity will travel in a helical path since the outside edge of the bullet is constrained by the bore. The first concept to understand is that of lateral throwoff. So why does this matter? Lateral Throwoff That will throw off both the center of gravity and the principal axis of inertia. Think of a bullet with a small void or impurity off the centerline. In real life, all bullets have both static and dynamic imbalance. Its CG is centered, but it's principal axis of inertia (Ip) is at an angle to the bullet's axis of symmetry. if they are not in line, the wheel will wobble. Think of a tire being balanced - the goal is to add weights until the wheel's principal axis of inertia aligns with the wheel's physical axle. Static balance is straightforward enough, but what is this principal axis business? That is, the bullet's center of gravity is on the bullet's axis of symmetry (static balance) and the bullet's principal axis of inertia is parallel to the bullet's axis of symmetry (dynamic balance).

Perfect bullets are balanced both statically and dynamically. So why not just pick a really fast twist and be done with it? It turns out the very spin that keeps our bullets stable also conspires to send our shots off target - it is possible to over-spin a bullet. A better calculator that uses a more involved method is available at JBM Ballistics. Unfortunately, it's not a very good method, but I include it here to show that longer bullets require faster twists, which is basically all you need to know. Where T is the twist in bullet calibers, and L is the length of the bullet in calibers. The simplest is the old Greenhill formula: For the mathematically inclined, a more thorough treatment of the math behind bullet stability can be found in Robert McCoy's excellent Modern Exterior Ballistics: The Launch and Flight Dynamics of Symmetric Projectiles - but only if you have a heavy duty math background (a math or engineering degree or equivalent knowledge).įor the less eager, there are several approximate methods of calculating the required spin for a bullet. Those constants are not generally available. Accurately calculating the amount of spin required to stabilize a given bullet requires the use of some empirically derived constants, not unlike the ballistic coefficient we're all familiar with. The longer the bullet, the more spin it needs. If you spin them at a high rate, however, they behave quite nicely, and will keep their nose pointed more or less down range. Left to their own devices, they will tumble through the air in an accuracy destroying, chaotic manner, hitting the target in a basically random orientation - straight, sideways, or even backwards. But how and why? Bullet Stabilityīullets are aerodynamically unstable. The barrel twist you choose for your rifle can have a significant impact on it's accuracy.

Base jumping line twist