Question: "Muzzle Energy" vs. "Power Factor"

[Dr. Bullseye]

We are talking about bullets here, right? The earth has momentum caused by its movement. But we never think of this unless we are launching a rocket. Muzzle Energy is the kinetic energy of the bullet flying through the air. If you step in front of a bullet, you are going to receive all (or most--it could fly all the way through you) at once. If you step in front of the earth's momentum you never notice it because you are traveling with the earth at the same speed (unless you are entering earth's orbit from space as an astronaut, going West to East). So for our discussion and as a matter of practicality, the kinetic energy produced is a better measure.

[Todd.K]

So for our discussion and as a matter of practicality, the kinetic energy produced is a better measure.

Neither is useful. Energy does not tell us anything useful about terminal performance.

Examining the damage done to something that closely simulates tissue is how terminal performance is determined.

[T2C]

Neither is useful. Energy does not tell us anything useful about terminal performance.

Examining the damage done to something that closely simulates tissue is how terminal performance is determined.

I agree. Our issued duty ammunition had a high calculated energy factor, but did not perform well on people I've seen shot with the cartridge.

[turnburglar]

The rules of a gun fight are as follows:

1. shot placement

2. shot placement

3. shot placement

4. capacity

5. expanding

6. fragmenting

7. follow up with additional shot placement



That's it.

[WillBrink]

Is it the greater power factor of .458 (600 kgr-ft/s) vs .308 (463 kgr-ft/s)? Or is it something else -- sectional density? Bullet weight and diameter? Expansion and penetration? What am I missing here?

Neither of those in isolation gives terminal performance info. End the day, that's the terminal ballistics of the bullet: velocity, mass of the projectile, sectional density, etc, are variables dictating that behavior in tissues: how much actual damage is done. I believe that's an accurate, if not overly simplified, summary of it.

[Dr. Bullseye]

The imparting of kinetic energy to an object is measured in joules

https://en.wikipedia.org/wiki/Joule

and is exactly what we are talking about.

The joules imparted from an AK round is higher than an AR round, for instance.

[MegademiC]

The imparting of kinetic energy to an object is measured in joules

https://en.wikipedia.org/wiki/Joule

and is exactly what we are talking about.

The joules imparted from an AK round is higher than an AR round, for instance.

A 115gr 9mm is about 400ft-lb
A 147gr hst is about 300ft-lb

Does that mean 115gr fmj is more effective?

[Todd.K]

The joules imparted from an AK round is higher than an AR round, for instance.

And provides nothing useful to compare terminal performance.

[AndyLate]

It is a mistake to dismiss energy as being unimportant. Terminal performance/FBI tests are valuable in selecting the best load for a given cartridge, not for selecting the cartridge itself.

The best performing round in a .380 ACP (200 ft/lbs) is not going to exhibit the same terminal effect as the best performing 9x19 (330+ ft/lbs).

Unfortunately, energy calculation favors velocity, so you have to take it with a grain of salt - the 7.62x25 (Tokarev) cranks out 500+ ft/lbs of energy while delivering sub-optimal terminal performance.

Andy

[LimeSpoon]

Mass and velocity can be useful to help predict terminal performance, but if the terminal performance is already known, then external predictors are not important.

Gel tests can certainly be used to compare different calibers; we can, for example, conclude that X 9mm loading performs better than Y .40 S&W loading, regardless of energy or momentum figures. One could also guess that the best performing .40 loading would probably have better terminal performance than the best 9mm loading, and they'd likely be correct - but without testing to actually verify this, it remains just a guess.