About Ballistics

Ballistics is the study of the path of a projectile and how it is affected by external factors (gravity, air resistance, wind, etc). Exterior ballistics is the sub category that pertains to rifle and pistol bullets that are fired at a relatively low angle relative to horizontal and have no means of propulsion after leaving the muzzle. Trajectory is a primary measurement of concern. It is the position of a bullet in relation to the line of sight (aim point) at any point along its flight path. When the bullet is above the line of sight, the trajectory is positive.  When it is below the line of sight, the trajectory is negative. Trajectory is extremely important to the hunter or target shooter because it determines where the bullet will strike compared to the point of aim.  The error in point of impact vs point of aim can be much larger than one might first imagine. This lesson is quickly learned here in South Texas where long shots are not uncommon. Ballistic coefficient is a measure of how well a bullet is able to overcome air resistance compared to other bullets. The higher the ballistic coefficient, the less air resistance impedes the forward motion of the bullet. A standard bullet was developed by military ordinance personnel who first studied the flight of projectiles (ballistics). It is a one pound projectile (bullet) that is one inch in diameter and has a semi-pointed shape. This standard projectile was defined as having a ballistic coefficient of 1.0000. The ballistic coefficient of other bullets are determined by the ratio of how far they must travel to lose the same amount of velocity as the standard projectile. Most hunting and target bullets have a ballistic coefficient less than one simply because they are not as long and heavy as the standard projectile and can not "punch" a hole through air as easy as the standard projectile does. Although some of the "very low drag" target bullets are getting pretty close. There are some heavy 50 caliber target bullets that actually have a ballistic coefficient greater than one.

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3. Sight-in distance - when a bullet leaves the muzzle towards the target, it spends about half the time rising and the other half of the time falling. When a target is beyond the sight-in distance, the bullet will strike below the point of aim. When the target is closer than the sight-in distance, the bullet will strike above the point of aim.

The two trajectory curves below illustrate this concept.

4. Air resistance - is not constant.  It is directly related to air density which depends on altitude, temperature, pressure, and moisture content. Increased altitude, temperature, and moisture content will reduce air resistance so that the trajectory will be "flatter". Increased pressure will increase air resistance.