The impact made by a bullet is influenced by the bullet diameter and weight, as well as its speed. Understanding the relationship among these variables allows the shooter to choose the proper combination of firearm and ammunition best suited for a specific purpose. The effect a bullet has upon impact may be defined in several ways, depending on how the information is used. For some purposes, it is useful to know the energy the bullet will deliver to the target, while other applications, such as bullet stopping or killing power, may benefit from more detailed calculations.

Collect the required information for the ammunition under consideration. You will need to know the bullet weight, measured in grains, which is listed on the ammunition box. You must also know the velocity the bullet is moving at impact. Each ammunition manufacturer publishes ballistic tables for their ammunition. These tables will normally list the bullet velocity at the muzzle of the gun and at intervals of 100 yards out to the useful range of the ammunition (usually 300 yards or so). For distances between the listed increments, you must estimate.

Ammunition manufacturers typically measure rifle bullet velocities using a 24-inch test barrel. As a rule of thumb, for every additional inch of barrel length beyond 24 inches, the velocity increases by 20 feet per second. Likewise, for every inch of barrel length below 24 inches, the velocity decreases by 20 feet per second. For example, if your rifle barrel is 20 inches long, then subtract 80 feet per second from the manufacturer's stated velocity. There is no such rule of thumb for pistol ammunition.

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You also will need to know the bullet diameter. Precise diameters can be found in tables published by ammunition manufacturers. Otherwise you can use the bullet caliber as an approximation. A 30-06 bullet has a diameter of 0.308 inches, but you could use 0.300 inches in your calculations without significantly compromising accuracy.

Calculate the energy the bullet will deliver to the target on impact using the formula Energy = Bullet Weight x Velocity^2 / 450,437. In words, the bullet energy (in foot-pounds) is equal to the bullet weight (in grains) times the square of the bullet velocity (in feet per minute) divided by 450,437.

Calculate the Hornady Index of Terminal Standards (HITS) number using the formula HITS = Bullet Weight^2 x Velocity / Bullet Diameter^2 / 700,000. In words, the HITS number is equal to the square of the bullet weight (in grains) times the velocity (in feet per second) divided by the square of the bullet diameter (in inches) divided by 700,000. Note that the HITS calculation method is meant to be an indicator of how severely a specific bullet might wound a live target, and today it is the most widely used calculation for this purpose. Other calculations used include the Optimal Game Weight (OGW) formula, the Taylor Knockout (TKO) formula, the Knockout Value (KOV) formula and the Lethality Index (LI) formula.

Hornady Manufacturing publishes tables of the HITS numbers for various types of ammunition, using a standardized range of 100 yards for rifle bullets and 50 yards for pistol bullets.