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Forum Index > Rifles general discussion > Kinetic Energy and Momentum? What force is most predominant for the lethality of a cartridge?

Kinetic Energy and Momentum? What force is most predominant for the lethality of a cartridge?

11 Jul 2024
@ 07:20 pm (GMT)


I recently opened a discussion about why some cartridges that have low energy but more weight and size are usually recommended for killing larger targets while cartridges with more energy but less size and weight have a more comprehensive recommendation, including targets and smaller size and weight. I used as an example the classic 45-70 in comparison to the 7mm RemMag and the 30-06 Springfield, where the 45-70 has low energy, especially above 100 meters in relation to the other 2 cartridges mentioned that have double or almost that.

The responses I got were unanimous in saying that the 45-70 is more recommended for larger targets, as it has more "momentum" to the detriment of the other two mentioned.

When studying this superficially, I realized that the "Moment" has a greater capacity for penetration, as it is a directed force, while kinetic energy has a greater capacity for "destruction" as it is an undirected force (inelastic).

And that brought me here, I want opinions on this subject in relation to the lethality of a cartridge. After all, what determines the lethality of a cartridge: Momentum or Kinetic Energy?

The more you support your opinion, the better it will be understood by me. After all, I am a ballistics enthusiast, with little knowledge but a lot of desire to increase and improve it.


12 Jul 2024
@ 07:33 am (GMT)

Nathan Foster

Re: Kinetic Energy and Momentum? What force is most predominant for the lethality of a cartridge?
Hi DR015, I have been writing about this for some time now, to be published in an upcoming book. The subject cannot really be covered in a short forum reply.

The question is whether momentum or kinetic energy has the greatest effect on terminal performance. The answer is neither. Without taking the bullet shape and construction into account, any such model is simply theory. These days, people are using theory to prove scientific concepts without taking facts or evidence into consideration. Science has become an ideology, bordering on a new religion.

The best way to determine the lethality of a cartridge is to test it on game. There is simply no substitute for this.

An example of the effects of bullet shape can be found in my article, The effect of the meplat on terminal performance.

Examples of the effects of bullet construction can be found throughout the knowledge base.

Otherwise, you have already identified basic factors such as momentum being key to penetration on large bodied animals.

See also:

The best way to summarize this is to quote past Hornady Ballistician Dave Emary. He has distilled this into one single accurate statement - "it depends".

13 Jul 2024
@ 01:42 am (GMT)

DR 015

Re: Kinetic Energy and Momentum? What force is most predominant for the lethality of a cartridge?
I read your article and managed to understand a good part of what was said, the small part that I couldn't understand was due to a lack of personal knowledge on the subject, although there is a lot of enthusiasm on my part to increase and deepen my knowledge about this area of ​​physics (as initially stated).

Just to give some “light”, even if superficial, on the subject in relation to the penetration capacity of a cartridge: As you said in your article, and also reaffirmed here, the lethality of a cartridge goes beyond its energy and momentum, which includes the “projectile type” factor. In this sense, is it possible for a cartridge with greater energy but less impulse to be more penetrating than a cartridge with opposite characteristics, given that the projectile has properties that seek this effect?
13 Jul 2024
@ 07:22 am (GMT)

Scott Struif

Re: Kinetic Energy and Momentum? What force is most predominant for the lethality of a cartridge?
Having had mixed success with various cartridges and bullets, as well as studying Nathan’s work for several years, I have concluded that attempting to develop a formula based on buzzwords such as momentum, sectional density, kinetic energy, and so on, is a fool’s errand.

There are 3 reasons such physical properties are next to useless in predicting terminal performance

1. Manufacturers of hunting bullets design them to perform consistent with customer expectations regarding external ballistics and terminal ballistics, neither of which is necessarily consistent with quick dispatch of game.

2. Manufacturers design hunting bullets to perform optimally (i.e., consistent with customer expectations) when shot placement is ideal, a criterion also dictated by customer expectation.

3. Shot placement on game is often not ideal.

Even within a line of hunting bullets, those of various calibers and weights may differ in construction, according to the manufacturer’s perceived view of customer expectation. Jacket thickness is an example. Manufacturers will vary it according to their best guess of what a customer expects from the bullet. “The deadliest mushroom in the woods,” is a prime example. Remington knew what their customer wanted: A shank with a mushroom lodged against off-side skin.
Momentum, sectional density, and kinetic energy have nothing to do with it.

The forums are replete with ad nauseum debates about the efficacy of certain bullets, the merits of pass-through, wasting energy on the hillside behind the game, blood trails, etc., all driven by customer expectation. A bad shot strikes a spine, the bullet is declared an engineering marvel.

The only apples to apples comparison possible is to compare match bullets of the same line but in different weights and calibers. Match bullets are constructed with the thinnest possible jackets, in order to ensure stringent manufacturing tolerances, and thus accuracy. This is the only place where momentum, section density, and kinetic energy mean anything with regard to terminal performance.

13 Jul 2024
@ 11:51 am (GMT)

Magnus Vassbotn

Re: Kinetic Energy and Momentum? What force is most predominant for the lethality of a cartridge?

In my experience, as long as you have enough penetration from whatever angle you shoot, high energy/ high velocity gives quicker and more dramatic kills than high momentum/ hi mass. Whether that penetration comes from bullet construction or high SD is sort of another discussion. But a slower, heavier bullet will for the most part make the animal just as dead, but not as quickly. Very generally speaking, with all the usal caveats. In practical terms - on medium sized abimals like red deer, pigs and most planes game, penetration/ momentum is rarely an issue with normal cartridges with normal bullets. Whatever that is...



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