Reloading Ammo For Big Game Hunting- Part 1
How a bullet kills is simple; when it’s fired from the reloading brass, it destroys the surrounding tissue and disrupts the function of the animal’s vital organs. This disrupts the animal’s nervous system and causes extreme blood loss, equaling shock to the animal’s entire system. This most often results in instant death, if the shot is placed in the right place. The more damage a bullet can cause, the more likely it is to produce a quick, painless death.
Shock seems most likely to occur when light framed animals are hit by high velocity bullets. The classic case would be when a small (say about 100 pound), relaxed, deer or antelope is hit by a 130 grain .270 bullet at short to medium range. Sometimes the animal goes down so fast that it seems as if the earth was jerked from under its feet. This is the "four feet in the air" effect. But if the same animal were frightened or excited before he was hit, he might run a considerable distance before expiring.
Measuring Energy and Killing Power
Kinetic energy, in it’s simplest definition, is energy a mass produces while in motion. Energy gives us an idea of how much power there is to initiate things like bullet expansion and penetration but does not guarantee that they will occur.
It is generally recommended that a small-bore rifle bullet suitable for medium size game be carrying about 800 ft. lbs. of kinetic energy when it hits. Energy is greatest at the muzzle as the bullet is fired from the reloading brass and diminishes as the bullet loses velocity. When the velocity reaches zero, so does the energy. But long before that the bullet has fallen below the recommended level of energy for reliably killing deer size animals. So, the practical hunting range of any cartridge is ultimately limited by how much energy remains. (It is also limited by other factors, for example trajectory, but that is another subject.)
Sectional Density and Penetration
Sectional density is among the key factors in determining penetration. Penetration is important because the bullet must get well inside the animal to disrupt the functioning of its vital organs. A bullet that fails to penetrate the fur, skin, muscle, and bone necessary to reach the vital organs is unlikely to bring an animal down.
Sectional density is a bullet's weight divided by the square of its diameter. Basically, a long thin bullet of a given weight penetrates better than a short, fat one. Which makes sense when you think about it. For example, if other factors (like impact velocity and bullet construction) are equal, a 150 grain .270 bullet will penetrate better than a 150 grain .35 caliber bullet.
Weight and Diameter
The bigger the bullet diameter, the greater the frontal area and the larger the hole it tends to make in an animal. 6mm/.24 caliber is the absolute minimum bullet diameter recommended for use on big game animals, and some experts would say .25 caliber is a more suitable minimum. Bullet frontal area has a positive correlation with killing power (other factors being equal). Also remember the bullet weight required for a kill shot increases as the bullet’s diameter increases.
Expansion and penetration
More important than initial bullet diameter is expanded bullet diameter. A bullet that does not expand generally imparts little shock to an animal's system and disrupts little tissue. Bullet construction is very important to killing power, both for rapid expansion and for the penetration required to reach the vitals of large animals.