Penetration seems to be a product of momentum or mass * velocity. Energy is the mass * velocity * velocity. This may cause some confusion.
Kinetic Energy (Ek)
Ek = mv2
p = mv
Where: m = mass and v = velocity
Measurement of Mass & Velocity
Mass is the measure of how much matter there is in a physical body; solid, liquid, gas or plasma. In the SI units, the international weights and measures body, mass is given in kilograms. Weight is mass accelerated in a gravitational field, in the US this is pounds. Bullet and powder weights are given in grains with 7000 grains to the pound. The acceleration due to gravity is about 32.17 feet per second per second (ft/22). The unit of mass in Imperial units is the ‘slug’. The formula is:
Mslug = grains/7000*32.17 = grains/225190
Acceleration due to gravity g = 32.17 ft/s2
Big Game Calibers
Ever look at dangerous game cartridges? The .45 to .50 caliber African numbers are all large caliber and moderate velocity numbers pushing 400-550 grain bullets out to under about 2400 fps. Big heavy bullets designed for little or no expansion to punch through thick hide and muscles and bone to reach vital organs. Generally, a quick humane kills demands major disruption of the central nervous system along with the cardio-pulmonary organs which are deep inside the body or behind thick heavy bone.
What is Momentum?
Momentum resists the slowing down of the mass and resists the slowing of a projectile passing through any medium. As mass is interrelated to inertia both mass and velocity are proportional to each other, inertia would contribute to momentum as it is a property of mass. Now once penetration is complete though, then perhaps the deciding factor is the transfer of kinetic energy. Imperial English units of energy is foot pounds, while momentum is pounds-second (lbf∙s) or the slug-foot per second.
Bullet Kinetic Energy
Energy is the capacity to do work. During penetration of a projectile it transfers energy to the target medium. The creation of permanent wound channel and the temporary wound channel is the work performed. Granted dangerous game cartridges are in the upper regions for kinetic energy. In game the idea is to break major bone groups, scramble the central nervous system and cause extreme trauma to the cardio-vascular network. Broken bones compromise mobility, headshots destroy the primary node in the central nervous system and heart-lung shots stop the circulation of blood and the transport of oxygen to the tissues.
Conservation of Momentum
Since momentum is the measure of motion, and the higher the momentum the more resistant to change of motion and depends on the mass (inertia) and velocity to drive the projectile in deep to reach and disrupt vital organs. With large heavy game this means punching through thick hide, heavy muscle and large tough bones. So, momentum is the measure of the quality that gets the bullet far enough to allow the transfer of kinetic energy. Momentum is a better guide line than energy even though the transfer of energy begins with the bullets contact with the game. Bullets don’t cut, they crush and brute force their way through the target. The expansion or deformation of the bullet aids in this crushing and causes displacement of tissue and liquid creating the wound channel.
Kinetic energy is the traditional way to rate a bullet’s performance but emphasizes velocity because of the squaring of the velocity. Momentum lends as much emphasis to either the mass or the velocity. Double either the mass or velocity and you double the momentum, double the velocity and your kinetic energy quadruples. Momentum vs Kinetic Energy; you decide.