Understanding Energy (force) of a bullet 

Force and Muzzle Energy

The Science

Now, let’s get down to some Science.  As I mention in my bio I am an engineer.  As such I like to look at data and understand it.  Generally speaking, the more force that hits a subject you are defending against the better it will stop, however there are many other things to consider, such as expansion, wound channel, cavitation, weight retention, and penetration. So, force is not everything, but let’s understand it a little bit more. 

If you are not into science, you can skip the next section.  If you want to learn Read on.

Physics

The energy that comes from a bullet is called Kinetic Energy.  Many manufactures put both Muzzle Velocity in Feet Per Second (FPS) and KineticMuzzle Energy in Foot Pounds (ft. lbs.) For ammo manufactures standards are made by a group called Sporting Arms and Ammunition Manufacturers Institute (SAAMI) these ensure that pressures and forces allowed in a firearm are safe for that firearm. 

The muzzle velocity is the speed at which the bullet leaves the firearm.  This is often measured in a standard full-size firearm.   The speed (velocity) is measured on a device called a chronograph. 

If given the muzzle velocity, we can calculate the Kinetic Energy by using the formula that follows.  This formula can be used anytime that the velocity of the projectile is insignificant compared to the speed of light and is the same formula used by SAAMI to calculate energy.  If you took science recently it uses SI units for all of its measures.  The firearms industry still uses imperial measurements.

Understanding this then we need to think about two things.  The Projectile has velocity in two directions, first the forward motion from the firearm, second that which is acted on by gravity.  We are able to use the approximation for gravity in the form of:

 If we take an example of a 9x19mm projectile. The muzzle velocity published is 1150 fps. Bullets mass are measured in grains (gr).  A grain is a that is 1/7000^th of a pound (lb). 

In order to calculate Kinetic Energy  in foot pounds we must convert grains into pounds.  This is done in the matter of conversion if we used a 115gr bullet it is converted to pounds by:

/science

Comparing Energy

I will save you the work of calculating and use published data from Speer about their Gold Dot Ammunition.

Let’s compare one specific brand 40 S&W to a 9MM in the same brand.

All of the Energy is within roughly 110 ft lbs between 40 and 9x19mm. Also notice when you hit the 180 grain its energy drops because its mass is so much higher.

The next biggest jump you would see is that of a 44 magnum which for a new shooter is not recommended.  44 magnum ranges around 900 ft lbs of energy. But it has an equivalently big recoil.

-Castle