The world of cartridges is full of numbers. Velocities, weights, diameters, coefficients and densities can make if very difficult to determine which rifle cartridge is the most powerful. Is a fast rifle cartridge the best? Or is a heavy bullet travelling slower more effective? By breaking down the numbers let’s determine a system to see which cartridges are the deadliest.
Kinetic Energy
The most basic measure of the power of a rifle cartridge is kinetic energy. Using basic high school physics, we can calculate how much energy the bullet is carrying. This is also the maximum amount of energy available to be transferred to the target upon impact. While this is not a bad metric, it doesn’t necessarily capture the entire picture.
If we think back to high school physics, we know E = 0.5xMxV2 where E is energy, M is the weight of the projectile, and V is the velocity on impact. This tells us that the kinetic energy increases exponentially with velocity but only linearly with mass. This means that the energy numbers tend to favour lighter, faster travelling projectiles even though they may not penetrate as well.
Kinetic Energy Calculation
For example we can look at the Hornady Precision Hunter loading in 30-06. There is a 178 gr bullet travelling at 2750 fps at the muzzle. This means that 178 * 27502 * 0.5 should give us our kinetic energy. However, we need to divide it by 225120 to convert it to imperial units. This gives us 2990 ft-lbs of energy.
Hornady Index of Terminal Standards
Hornady tries to address this with their Hornady Index of Terminal Standards (HITS) formula. This is calculated by multiplying velocity at impact with bullet weight and sectional density. The sectional density is a number that shows how heavy a bullet is for its diameter, which tells us how well it will penetrate. The higher the sectional density the better it will drive through tissue. Hornady divides their final number by 100 to make it a smaller number that is easier to compare.
The HITS number is a better measure, but it neglects to factor in the fact that velocity is more important in terminal performance than bullet weight if the penetration is adequate. That leads us to my favorite metric, developed by the late Chuck Hawks, which is the Killing Power Score. The KPS multiplies the energy of the bullet at 100 yards with the sectional density and the frontal area of the bullet. This metric is well thought out for a number of reasons.
Killing Power Score
First of all, it factors in the energy of the bullet at a reasonable distance. Since most shots at big game occur between 50 and 150 yards, taking the kinetic energy at 100 yards is a good estimate to determine how hard a bullet is actually hitting game. Next, it uses the sectional density to estimate penetration. Sectional density tells us how well the chambering will penetrate compared to different diameter bullets of the same velocity. Finally, it factors in the size of the hole made by the bullet. This seems obvious, but many formulas fail to factor in the size of the hole created.
In order for the numbers to have some relative value lets impose some arbitrary limits based off of killing power score. A killing power score of 12.5 or greater is suitable for medium game such as deer or black bear, a score of 40 or greater is suitable for moose or elk, and a score of greater than 75 is adequate for dangerous game. Let’s take a look at some of the most popular rifle cartridges on the market and how they stack up.
KPS Calculation
Lets go back to the Hornady Precision Hunter load from earlier. By performing the same calculation with a 100 yard velocity we get a kinetic energy of 2637 ft-lbs at 100 yards. The sectional density is just the weight in pounds divided by the diameter squared. So the weight is 178/7000 to convert it to pounds and then we divide that by 0.3082 which gives us a sectional density of 0.268. Finally we need the frontal area, which is the size of the hole a .308 diameter bullet makes. We divide the diameter in half to get the radius, then square the radius and multiply that by pi. This gives us (0.308/2)2 *pi. This means our frontal area is 0.075. Finally, we get our KPS of 2637*0.268*0.075 = 53
As we can see, out of these ten popular rifle cartridges the 223 Remington is best for varmints. Also, the 270 Winchester is just above the minimum to be a good moose and elk cartridge. Meanwhile, and the 300 Winchester Magnum is just a little shy of meeting the threshold as a dangerous game cartridge. Note that the KPS numbers generally agree with the kinetic energy numbers, however the KPS tends to favour rifle cartridges with very large diameter heavy bullets.
Concluding Thoughts
In conclusion, any formula that measures the power of a rifle cartridge should be used as a guideline, not a rule. Shot placement is the best predictor of cartridge effectiveness. The second most important predictor is bullet construction. It is more important to find a cartridge that you can shoot well than to hit some minimum power threshold. However, the KPS can help to give sportsmen an idea of which cartridges are appropriate for certain classes of game.