Questions about the irrelevance of energy (ft-lbs)

[NorthIdahoDude]

Most of the numbers that get involved in a discussion of terminal ballistic potential end up being used in discussions in terms of comparing cartridge one to cartridge two (which is kind of fair use), or (far more common) are used in isolation from all the other numbers which should be considered in conjunction with them to make them meaningful. However, that doesn't make such things useless, it just means we have data we aren't usually using for what it's good for.

What kills animals, is sufficiently damaging something that yee olde animal cannot live without. To do that, shot placement (hitting a thing that cannot be lived without) and penetration (putting a hole in that thing) are hard requirements. Everything else is just trade-offs we make for (sometimes) good reasons.

You can make trade-offs to get either more penetration (low expansion/high weight retention such as Barnes/Mono's/etc), or to a wider wound tract (expanding and/or fragmenting bullets like the ever-popular Accubond, ELD-X, etc, etc).

To go at one aspect of that in particular, we can hedge our bets with regards to marginal shot placement by widening our wound tract with a fragmenting and/or expanding bullet, but we will loose penetration depth by doing this. Too little penetration, and we loose one of our 'must-have's, but, we can easily hedge our bets against having too little penetration by using a bullet with sufficient mass to allow some reasonable percentage of the bullet to fragment/expand a lot and still leave enough mass traveling at enough velocity for sufficient penetration.

Of course, if you're familiar with physics, you will instantly recognize that Mass and Velocity are how we calculate Kinetic Energy - so yeah, true story, we do want sufficient 'energy'. However, among hunters, the term 'energy' usually gets kicked around in context of 'you need X amount of energy to hunt elk (or moose, or deer, or whatever)', which is a totally irrelevant way to look at it. So yeah, in that kind of discussion, energy is utterly and totally irrelevant.

But energy is relevant (required even) if you wanted to fully calculate out that you need X amount of energy, to cause bullet of mass Y, with expected retained mass/fragmentation ratio of Z, with an impact velocity of A, and an expanded diameter of B, to penetrate C inches of critter D, through E inches of bone, and F inches of muscle, with G margin for error, and...

You get the idea - mass and velocity (aka: energy) are indeed part of the math by which we could ultimately come to a fairly accurate guesstimation of the answer to the question, "is this gonna git er dun, or do I need More Gun(tm)?", but used outside of that context, particularly when used as a standalone measure of killing ability as hunters almost always do, calling it irrelevant in that context is spot-on.

 

[Thegman]

I feel like maybe I'm not understanding the way you're wording this but it seems essentially backwards. Momentum is conserved (meaning that the combined momentum of the bullet/gases/powder and the momentum of the rifle are equal magnitude, opposite direction) but in real world inelastic interactions like this kinetic energy is not conserved. In fact, two objects of different mass by definition cannot have both equal momentum and equal KE. Definitionally, a lower mass object will have more KE than a higher mass object with equal momentum.

The bullet ends up with slightly less momentum than the rifle (not more) in a free recoil scenario since some of the "equal and opposite" momentum with the rifle is shared between the bullet and the powder/gases (though I suppose that by the end of the event some of the gas at the breech end of the barrel is headed rearward with the rifle).

The force imparted on the bullet and the rifle are nearly the same (at the beginning of the burn the force is being applied to rifle in one direction and the bullet plus remaining powder column in the other). Obviously equal force accelerates the less massive object at a higher rate. That doesn't mean the KE gets "used up" "overcoming inertia" for the heavy rifle but not for the light bullet. An object has the same inertia whether it's in motion or stopped. The force applied to the base of the bullet accelerates it in one direction, and the (equal) force applied to the bolt face accelerates the rifle in the opposite direction, at a rate directly proportional to the ratio of the masses of the two objects.

If I'm misreading what you're saying (and/or not being clear in what I'm describing), I apologize.

Edit to add: Your posts (especially as they relate to physics) are generally pretty right on from what I can tell, and my assumption is that there's a communication breakdown and not a fundamental understanding problem causing me to think your description of what's happening is not correct. Not sure if that breakdown is on my end or yours, or maybe some of both.

And, we have a winner! This is the actual physics, folks.

 

[BigCountry49]

I am well aware of the differences between energy and momentum. This is ultimately why it is difficult having this conversation with people who's understanding of physics comes from gun rags and ammo boxes (comment not aimed at you, just folks on gun forums in general).

The statement that I refuted was "the bullet receives more energy due to it having a smaller mass". That is simply not correct. Accuracy of wording is crucial when having these discussions, because as you have pointed out, there is a significant difference between energy, momentum, mass, weight, etc.

The amount of KE imparted to the bullet and the rifle are the same. However, in order to overcome inertia a lot more of it is used up to convert that energy to motion when talking about the rifle. So the amount of acceleration for the bullet is much higher than it is for the rifle (Newton's 2nd law). The result of this is that the bullet ends up with more momentum than the rifle, but energy imparted to both of those objects is the same.

Can you explain why the top response to this incorrect?

Why does the bullet have greater KE than the rifle?

A rifle is fired, and the bullet moves much faster than the rifle, and momentum is conserved. My question is whether the kinetic energy of the bullet is greater than the kinetic energy of the rifle

physics.stackexchange.com

 

[nm.otter]

But energy is relevant (required even) if you wanted to fully calculate out that you need X amount of energy, to cause bullet of mass Y, with expected retained mass/fragmentation ratio of Z, with an impact velocity of A, and an expanded diameter of B, to penetrate C inches of critter D, through E inches of bone, and F inches of muscle, with G margin for error, and...

So we have ten variables and what, one or two equations? Literally unsolvable.

 

[SteveAndTheCrigBoys]

So we have ten variables and what, one or two equations? Literally unsolvable.

I know the right guy for the job...

 

[NorthIdahoDude]

So we have ten variables and what, one or two equations? Literally unsolvable.

Because unknowable physics comes into play each time a bullet strikes a flesh and bone target, the idea of coming to a full and definitive solution was a given before we ever started this exercise, LOL. Even if energy was the only thing that mattered, penetration, deflection, etc will always vary to some degree due to the nature of the thing.

However, I would still maintain that coming up with a good GUESS-TIMATE is pretty doable, by considering the factors I mentioned (and a few others) in correlation with each other, and that's not quite as rocket-science-y as I made it sound. I was just trying to hammer the point that KE value as it's typically used (eg: bubba said "you need 1500 lbs of energy for an elk, minimum"), is truthfully irrelevant and useless.

For a guesstimate example, I would guesstimate that a 110 grain 308 Varmint bullet out of a 30-06 at say 3500 FPS impact velocity, stands a very high chance of not producing deep penetration in an animal like an elk which has thick skin, dense muscle, and more substantial bones (than say a deer). Doesn't mean it won't kill it, just means it won't penetrate that deep which likely won't matter one whit if you neck shoot him or perfect-broadside him behind the shoulder, in fact it will have such a wide wound tract, you'll get a lot of margin for error in shot placement on those two scenarios ... but if you take a hard quartering away shot and need the bullet to go through 2 foot of elk before it gets to the heart, that likely won't work out so well. Slow that same bullet down to say 1800 FPS, and while it still won't penetrate as crazy-deep as our next example below, it will penetrate a lot deeper than it would at 3500 FPS because of much reduced energy acting on the bullet to tear it apart. So, yeah, we are absolutely using energy in our guesstimation here.

Conversely, let's say a 55 grain TTSX, half the weight of the 110 30 cal varmint bullet above, pushed to 3500 FPS out of say a 22-250, should have a LOT of penetration, even on a big animal like an elk, but since the wound tract is narrow, our margin for shot placement is reduced, but our options for shot angle compared to the above just increased significantly.

Anyway - back to my point - KE is not a useless number, it's just not useful in the way it gets kicked around in hunting circles in terms of how much you need (or don't need) to kill any given animal.

 

[Thegman]

Because unknowable physics comes into play each time a bullet strikes a flesh and bone target, the idea of coming to a full and definitive solution was a given before we ever started this exercise, LOL. Even if energy was the only thing that mattered, penetration, deflection, etc will always vary to some degree due to the nature of the thing.

However, I would still maintain that coming up with a good GUESS-TIMATE is pretty doable, by considering the factors I mentioned (and a few others) in correlation with each other, and that's not quite as rocket-science-y as I made it sound. I was just trying to hammer the point that KE value as it's typically used (eg: bubba said "you need 1500 lbs of energy for an elk, minimum"), is truthfully irrelevant and useless.

For a guesstimate example, I would guesstimate that a 110 grain 308 Varmint bullet out of a 30-06 at say 3500 FPS impact velocity, stands a very high chance of not producing deep penetration in an animal like an elk which has thick skin, dense muscle, and more substantial bones (than say a deer). Doesn't mean it won't kill it, just means it won't penetrate that deep which likely won't matter one whit if you neck shoot him or perfect-broadside him behind the shoulder, in fact it will have such a wide wound tract, you'll get a lot of margin for error in shot placement on those two scenarios ... but if you take a hard quartering away shot and need the bullet to go through 2 foot of elk before it gets to the heart, that likely won't work out so well. Slow that same bullet down to say 1800 FPS, and while it still won't penetrate as crazy-deep as our next example below, it will penetrate a lot deeper than it would at 3500 FPS because of much reduced energy acting on the bullet to tear it apart. So, yeah, we are absolutely using energy in our guesstimation here.

Conversely, let's say a 55 grain TTSX, half the weight of the 110 30 cal varmint bullet above, pushed to 3500 FPS out of say a 22-250, should have a LOT of penetration, even on a big animal like an elk, but since the wound tract is narrow, our margin for shot placement is reduced, but our options for shot angle compared to the above just increased significantly.

Anyway - back to my point - KE is not a useless number, it's just not useful in the way it gets kicked around in hunting circles in terms of how much you need (or don't need) to kill any given animal.

This is what I see a lot. You talk about KE being important for the "guesstimation", yet use bullet design and impact velocity for predicting outcomes, not KE, at all.

Like I've said before, yes, underneath it all, KE is what allows all of it to happen, but it's not useful in predicting what will actually happen. Without knowing bullet design and impact velocity, no prediction can be made. Once those two things are known, KE still isn't used in the prediction.

 

[NorthIdahoDude]

This is what I see a lot. You talk about KE being important for the "guesstimation", yet use bullet design and impact velocity for predicting outcomes, not KE, at all.

Like I've said before, yes, underneath it all, KE is what allows all of it to happen, but it's not useful in predicting what will actually happen. Without knowing bullet design and impact velocity, no prediction can be made. Once those two things are known, KE still isn't used in the prediction.

That's relatively fair, and I think we're kind of ion the same page with that. I am technically still using KE indirectly by taking impact velocity and bullet weight (mass and energy) in consideration in correlation with bullet design, but it's absolutely fair to point out that I am not specifically using "KE value of X" in my estimate directly.