Measuring Effectiveness of Cartridges: Hornady Index of Terminal Standards

First, let me wish the faithful readers of this blog a happy 2013.

Continuing our series on measuring effectiveness of cartridges, the next method we will study is another empirical method, this one originates from a cartridge manufacturer. We're talking about the Hornady Manufacturing Company (a well known manufacturer of ammunition and handloading components here in the USA) and the formula in question is the Hornady Index of Terminal Standards, otherwise known as H.I.T.S.

The Hornady Index of Terminal Standards is intended to be a guideline for hunters to compare cartridge and bullet combinations for any hunting situation around the world. The Hornady company has made an online version of the calculator available on their website, for the benefit of hunters. By looking at the Javascript code behind their webpage, it is not too hard to figure out what formula they are using. The formula is essentially:

HITS = (W² / 7000) * (V/D²) * 1/100

where
HITS = Hornady Index of Terminal Standards
W = Weight of the bullet in grains.
V = Impact velocity of the bullet in feet/sec.
D = Diameter of the bullet in inches.

The actual formula on their webpage does a bit of rounding here and there, but that doesn't change the result of the formula too much. Hornady claims that the HITS formula factors in bullet weight, sectional density, ballistic coefficient and impact velocity. Hornady then classifies the HITS value calculated by the formula above into one of four different classification types. The classifications are as follows:

H.I.T.S	Suitable For
below 500	Small game animals that weigh less than 50 pounds (22.67 kg.)
501-900	For medium-sized game animals weighing between 50 to 300 pounds (22.67 - 136.1 kg.) Suitable for Antelope, Deer, Black Bear, Caribou etc.
901-1500	Suitable for large and heavy animals which are generally not considered dangerous, weighing between 300 - 2000 pounds (136.1 - 907.2 kg.) This list includes elk, moose, red stag, American bison, zebra, kudu, giraffe and other such African plains game animals.
over 1500	Suitable for animals that are considered dangerous game i.e. animals that have no problem stalking the hunter. This would include lions, tigers, leopards etc.

Let's take the same rifle that we considered when we calculated the Thorniley Stopping Power a few articles earlier. We assumed a .30-06 rifle (such as the M1903 Springfield rifle or the M1 Garand rifle) firing a bullet weighing about 180 grains and .308 inch diameter moving at around 2900 feet/sec. Plugging the numbers into the formula above, we have:
HITS = (180² / 7000) * (2900 / 0.308²) * (1/100) = 1414.957 approximately

Looking at the classification for this HITS value, we see that this value falls in the third category, i.e. it can be used to shoot large and heavy animals that aren't considered dangerous e.g. elk, moose, red stag, zebra, kudu etc. This is similar to the conclusion we reached when we calculated the Thorniley Stopping Power value for this same cartridge/bullet combination.

Note that the HITS values are empirical and Hornady only classifies the HITS value into four different general categories. A HITS value of 800 doesn't mean that it is twice as powerful as a HITS value of 400, for instance. Hornady says that their HITS rating for their own ammunition is based on measuring impact velocities at 100 yards distance for rifle/muzzleloader/shotgun bullets and 50 yards distance for pistols/revolvers.