Topic of the Month: July 2002

The Taylor Index - A Measure of Stopping Power

We sometimes get a question such as "How do I compare the effectiveness, of one cartridge, say a .450 Marlin over a 30-06? I want to purchase a deer/bear rifle for woods and need some guidance."

Hunters and shooters have been searching a long time for the magic formula that will predict what is needed to drop an animal in his tracks, every time. No such formula has been developed, but this doesn't keep people from trying. Many thought that the kinetic energy of the bullet was a reasonable measure of the stopping power of the bullet and most bullet and ammunition manufacturers catalogs provide charts that show the velocity and kinetic energy of a given bullet anywhere along its trajectory.

The equation for this energy calculation is:

KE = m*v2/2 = bw*v2/450436 (1)

Where:
KE = Kinetic Energy, ft-lbs
m = bullet mass, slugs
v = bullet velocity, f.p.s.
bw = Bullet weight, grs.

Noted big game hunter and writer John Taylor thought that the kinetic energy equation gave too much credit to the new high velocity cartridges. Taylor, who spent a good many years in Mozambique, did a great deal of shooting and some commercial ivory hunting. He wrote a book entitled African rifles and Cartridges in which he pushed the British big-bore viewpoint. He thought these new cartridges, which fired a lightweight bullet at high velocity, gave too much importance to bullet velocity. Looking at the kinetic energy equation it can be seen that a small increase in velocity means a large increase in the kinetic energy because this energy increases as the square of the velocity. He thought that this would lead the hunter to believe that a small bullet, at high velocity, would be more effective on big game than a slower, heavier bullet. He stated that based on his hunting experience this was not so.

In an article in the November, 1947 issue of the American Rifleman magazine, Taylor gave several examples of how the heavier bullet knocked big game animals cold while the lighter faster bullet, in many cases, only dazed the animal for a few seconds. Taylor's energy equation is listed below:

TI = bw/7000*v*DI (2)

Where:
TI = Taylor Index
bw = Bullet weight, grs
v = bullet velocity, f.p.s.
DI = Bullet diameter, inches

Taylor's equation includes the bullet diameter, velocity and bullet weight. Since the velocity term is not squared it has less impact on TI energy as compared to the kinetic energy equation.

Table 1.0 shown below lists several popular cartridges and the generated kinetic energy (K.E.) and Taylor Index (TI).

Table 1.0

Note: All velocities and energies shown were calculated using the LFDW program.

From Table 1.0 it can be seen that the 450 Marlin develops only about 92% of the 30-06's kinetic energy at 100 yd, but the Taylor Index for the 450 Marlin is almost double that of the 30-06. Based on this information and the fact that heavier bullets usually work better in brush, the 450 Marlin looks like a winner. The last cartridge listed, the 50 BMG, is not considered a sporting cartridge, but was included to show how much energy this military cartridge can develop in comparison to other cartridges.

The Taylor Index is a relative value, which means the value has no units of measurement like ft.-lbs. or feet per second. It is more of an indicator of how one cartridge stacks up against another. This assumes, in all cases, that a well placed shot will be delivered to the vital area using a properly constructed bullet that will transfer all its energy to the game animal.

So, you might ask, "How do I use this information?" What we need is a chart of the Taylor Index figures rated against different type/size game. Listed below is my evaluation of such a table. Your ideas may differ and that's fine, but it is a starting place.

Table 2.0

Of course we all know that a 22 Long Rifle can kill a deer just as dead as the 416 Rigby, but how close did you have to get to fire a clean shot and how long did you have to track the deer before it dropped? By using the TI values from Table 2.0, as a guide, the hunting experience should be more pleasant for the hunter and more humane for the game animal.

Watch our web site for the next topic of interest. Until then, shoot safely and know where you bullets are going.

Sincerely,
The Ballistician

< Return