Kinetic Energy, Momentum, & Knockdown Power

First, we need to agree upon terms.  These are what these terms mean, and how they will be used for the rest of the post.

Kinetic Energy = (1/2)*mass*velocity², and is measured in foot-pounds. (a grain is 1/7000th of a pound)

Momentum = Mass*velocity, and is measured in pound-mass-feet per second

Knockdown Power = Fantasy, and is unmeasureable.

So, what do these mean?  Kinetic energy is the usable energy contained within a moving object.  A stationary object has no kinetic energy.  Momentum is also contained within moving objects, and stationary objects have none.  Knockdown power is contained within gun shop fairy tales, also known as “Fuddlore”.

EFudd

 

Calculations

Warning: Math ahead!

Let’s look at some example bullets.  Bullet 1 has a mass of 100 grains (gr) and is traveling at 1,000 feet per second (fps).  Bullet 2 has a mass of 200 gr and a speed of 1,000 fps.  Bullet three has a mass of 100 gr and a speed of 2,000 fps.  Bullet 4 is a 25% increase on both speed and mass from bullet 1.  Which has more kinetic energy, and which has more momentum?  Look below and find out.

1: 100 gr @ 1,000 fps = 222 ft*lb & 14.28 lb*ft/s  [a little hotter than a defensive .380 load]

2: 200 gr @ 1,000 fps = 444 ft*lb & 28.57 lb*ft/s  [a subsonic .300BLK load, or hot .40 S&W]

3: 100 gr @ 2,000 fps = 888 ft*lb & 28.57 lb*ft/s  [a little weaker than a .25-45 or .277 Wvn]

4: 125 gr @ 1250 fps = 434 ft*lb & 22.32 lb*ft/s  [a standard NATO 9mm load]

So, what does the info tell us?  That if you want to increase kinetic energy, you’re best bet to do so quickly is to increase velocity.  Increasing momentum is less nuanced, just add more speed or weight, it really doesn’t matter.  Also, that KE and momentum do not increase at the same rate as you add speed, but they do as you add weight.

But, why would I want to increase kinetic energy in the first place?  Because kinetic energy is what the bullet uses to influence the world around it, aka, wound.  More kinetic energy means that a projectile has more energy with which to inflict wounds to targets.  More KE = more wounding potential, less KE = less wounding potential.

Notice I said wounding potential, and that’s for a reason.

Think of kinetic energy as horsepower in a car.  More power means more better, right?  Well, your car needs to get that power to the ground (if you can think of another analogy for non-car people, let me know in the comments below).  So, where in this analogy does the rubber meet the road?  Bullet construction.  Proper bullet construction allows a projectile to transfer that KE into the target.  Just like a souped-up hot rod needs good tires to make use of the engine, poor bullet construction wastes a projectile’s kinetic energy.  Knockdown power is the equivalent of painting flames on the side of a car to make it go faster.

stopping power“muh knockdown power”

So, why did I even mention momentum at all?  Because momentum is the source of felt recoil.  Sir Isaac Newton famously described it in his laws of mechanics, “For every action, there is an opposite and equal reaction.”  The momentum of the bullet going forward is the same as the momentum of the gun going backwards, into your hand.  You calculate the momentum of your bullet, divide it by the weight of your gun, and that’s the recoil of the cartridge.  The shape and ergonomics of the gun itself, even the type of action of the gun, influence how that recoil gets transferred to your hand, hence the usage of the term ‘felt recoil’.  You can read more about that here.

A key thing to keep in mind is that kinetic energy and momentum do not increase at the same rate when increasing velocity.  They do increase at the same rate when increasing mass.  This means that 2 projectiles, one heavy and one light, can have the same kinetic energy, but have different momentums.  Both of these rounds would have the same wounding potential, but one would recoil harder than the other one.  Or, you could also have two cartridges that recoil the same, but one has a higher kinetic energy than the other.  Though both of those loads would kick the same, they would not be equals.

Adding speed and adding weight both make a gun recoil more, but adding weight increases the recoil faster.

The different bullets would also react differently to the target, but that’s a discussion for next week.

Conclusion

Seriously, stop using ‘knockdown’ or ‘stopping’ power.  It’s stupid.

Next week we’ll discuss the highlights of bullet construction.

-S_S

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