Is energy (ft-lbs) the best measure of the killing power of a round? We all know shot placement trumps all and bullet construction is very important but that aside how do you best compare the killing power of different calibers?

For example a 45-70 with 325gr bullets has 2,158 ft lbs at 100yds and is going 1,729 fps. Now compare that to a 308 with 178 gr bullets which has 2,348 ft lbs at 100 yards and is traveling at 2,437 fps.

We know the 308 is flatter shooting and better at long range but ignore that. At 100 yards which has more killing power? The 308 because it has more energy or the 45-70 because the bullet is nearly twice as heavy and much larger in diameter?

momentum is mass times velocity squared.

So, momentum (ft lbs of energy in bullet talk) would be approx 1,057,136,482 (just doing straight numbers don;t know how this equates to ft lbs) for the 308

for the 45-70 momentum would be 978,568,325

Just in sheer numbers the 308 should have more momentum (ft lbs of force)

But we all know bullet design and characteristics will have a huge play into "killing power". If the 45-70 dumps near 100% of that momentum into the animal while the 308 only dumps 75%, then the 45-70 would win the game. If the 308 dumps majority of the momentum into the animal then it will win. And the "dumping effect" is all about bullet design and performance.


Does that make sense? the short answer is it depends on what kind of bullet is being used by both at that range. Go further out and the velocity part will play a significant role in helping the 308.

There are so many variables it is hard to draw any conclusions from experiences but I have recently been shooting a 45-70 at hogs and it seems to drop them way better than my 308 or 6.5 PRC does. Both these rounds have considerably more energy than the 45-70 which got me wondering. I use ELD-X bullets in the 308 & PRC and FTX in the 45-70.

Again, physics is physics, and momentum is momentum, energy into target is more complex due to no perfect way to figure the amount if energy dumped into the target.

Shot placement is huge, but bullet performance at the target is next biggest factor in how much damage will be done to the target. Your question is answered by yourself, you see more dropped hogs with one vs the other, so in your system, the 45-70 is dumping more of its energy into target.

I don’t know but after 30 years of trial, to me and my eyes the .308 hurt the whitetails more than the .270 I used for a long time

I killed a lot w both, that’s just what my eyes said

It was take an exact shot placement on an identical animal to truly know or have an idea I think

Momentum is M*V
ke is 1/2 (M * V^2)

KE is simply a measure of max potential efficiency.

When it comes to archery it’s a measure of energy transfer from limbs to arrow. In rifle it’s measure of max potential energy but the bullet design dictates when and where that energy is transferred

So theoretically the highest energy round that doesn’t pass through the animal has the potential to do the most damage?

This is what I have been preaching for years, you just did a heck of alot better job putting it into words than I can.

This is why when I hunt one of my bow stands I go with something big and slow like a 444 Marlin over something far superior in more ways than not like a 270. A 270 dumping 20% of its energy in the animal aint the same as a 444 dumping 100% of its energy in the animal.

No, just has the most potential to transfer that energy, but that doesn’t mean terminal damage

If it dumps all its energy at the skin level, that bullet never makes it to the vitals, it may knock animal down but it gets up and leaves with a skin damage and maybe a broken bone. Where as the bullet stays intact until after skin and muscle then opens up to create a wound channel, that animal dies


22-250 can penetrate through 1/2 steel but a 308 won’t. Yet alot of people think the 22-250 is sub par. Ke and momentum are only potential indicators, surface area, sectional density, impact speed and bullet design play a huge part as well

And this is why i got a D in college physics….good catch.

KE is the value i was referring to, not momentum. Thanks to BOBO for clarifying that. So in everything i wrote above, just take momentum and replace with Kinetic Energy…carry on!

Another great break down of how KE does not necessarily translate to dead animal. Its about how the bullet dumps it’s energy and the bullet performance on/in the animal. I tried to explain that but BOBO’s explanation is better for terminal issues.

Yup, both are just measurements that a lot of people put a lot of stock in like 1000lbs to kill a deer and 1500lbs to kill an elk which is rubbish to me other than talking points and comparison.

A 22-250 has more KE than a 45-70 with some loads, but if faced with a mad grizzly which would you want in your hands?

Momentum is a better indication for hunting application IMO but it can also be misleading. Momentum is not the end all either a train moving at a couple miles an hour has more momentum than a bullet.

Taylor Knockout Formula is another that is used and has merit as well. It does give a weighting to the bullet diameter as well which can come into play especially for real big critters.

Taylor KO value = (Bullet diameter X velocity X weight in grains) / 7000 and is expressed i TKO value in pounds.

For any of you who are not familiar with the Taylor TKO, this will give you some interesting history and reading about the subject:

https://en.wikipedia.org/wiki/Taylor_knock-out_factor

Learned something new today I’ve never heard of Taylor KO value. Interesting read. Thank you.

Grizzly for me is just weird thought. I know I have one shot to get it in his head, maybe two if I have a doubles. Hit guts and even most vitals I’m probably dead or close to still regardless of caliber

I have never heard of that either. Thank you Kmon for dropping some knowledge on this.

Easy calculation for TKOF, KE and Momentum: https://1895gunner.com/taylorknockout.html

The problem with "energy" is that there is no calculation for energy and damage. Coroners often have a hard time telling what bullet was used on a person, if they can't find the bullet, because the wounds often look so much alike. The same goes for shooting hogs, deer, bear, etc. Nobody ever looks into the wound cavity and says, "Yep, that looks like it was hit with about 1150 ft lbs of energy. You can tell by such-n-such. If it was over 1300 ft lbs, you would see this-n-that. If if was under 1000, it would look more like some other stuff."

I have yet to see anybody do this, but I assume it could be done. It would be neat to measure the volume of the permanent damage done inside an animal or inside a gel block. Gel blocks are only going to give you a hypothetical ideal as they are perfectly uniform and while the bullet may behave in a consistent manner in get, get is not the same as actual tissue. Case and point, I can stick my finger through undamaged gel and recover a bullet. I can't stick my finger through undamaged muscle tissue and recover a bullet.

Ideally with gel, you would get the same volume amount of permanent damage for the same amount of impacting kinetic energy whether you used a FMJ or some sort of expanding round. You might have to string 2-4 blocks together to stop some FMJ bullets, but their long, tubular wound channel should be the same volume as that of the much shorter wound channel of an expanding bullet...in an ideal world.

Animals are not get blocks. Tissue densities differ and you have air pockets (lungs), fat (largely amorphous, most like gel), muscle, tendons, ligaments, and bone of various thicknesses and densities. The permanent would channel that involves striking a bone is potentially going to be smaller than one that doesn't strike bone because of the energy needed to break through the bone is more than the energy needed to go through a comparable amount of soft tissue. I have see bone stop bullets that otherwise would have penetrated farther.

However, hitting bone can sometimes create significantly more damage in a limited area than might have been attained hitting only soft tissue with the bullet. The bone fragments can become shorting distance projectiles themselves, radiating from the point of impact in the general direction of travel or deflection (usually). Further compounding the issue is that you aren't necessarily going to see the same result from a very similar shot in the same place hitting the same bone on a different individual animal. As such, there are NO calculations for this.

Some people want to go off the notion of "what kills hogs/deer/bears/ better" sort of mentality and often confuse the notion of one kill being better than another based on how far an animal ran. This isn't very telling unless the animal doesn't run at all, or runs really far. You can shoot the heart or lungs out of a hog or deer and still have it travel 100 yards. Sometimes they travel 5, 50, or 75, or 100. They were all equally killed and the difference can be more so on the health and status of the animal at the time of the impact more so than bullet performance. They are going to live until they run out of oxygen to the brain, either due to suffocation or exsanguination.

This is all direct damage. Then their is the issue of indirect damage. You have hydraulic shock that is often referred to as the temporary wound cavity. That is local to the wound channel itself and is not known to cause much in the way of permanent damage to animals because most soft tissue is fairly elastic and hard tissue doesn't usually break when subjected to it. I have see hydraulic shock to the spine (bullet did not hit the spine itself, but the permanent wound cavity passed right below the spine) stun an animal that then got up and ran some minutes later. Next you have hydrostatic shock or the remote wounding of critical brain or brainstem structures by shots elsewhere in the body. This is a huge benefit when it occurs, but does not occur in a predictable manner. Larger, heavier, and faster calibers (and this is seemingly relative to animal size) seem to create hydrostatic shock instant incapacitation more often than smaller, lighter, and slower calibers. For example, I have seen hogs shot with expanding bullets from everything from .223 to 300 Win mag where the shot hit the shoulder area without hitting the spine and the animal went down and stayed down and also those that ran after the impacts. If I was to plot out what I thought was going on, I could say it happens less often with the .223 than with the .300 Win Mag.

I will add one last comment that will further muddle the issue. When looking to calculations to ascertain killing potential, I do not believe that the results are directly linear. With kinetic energy, I highly doubt you would be able to determine any sort of significant killing potential difference with similar shots made on similar sized (let's use) 200 lb hogs whether they were hit with 1000 ft lbs or 1100 ft lbs using similar bullets. Too many other variables are involved. Now, if you are comparing 300 ft lbs to 1000 or 1000 versus 2000, markedly different amounts, then you could see some real categorical differences in lethality performance. Then again, there comes a point where a 2" permanent would channel through the body isn't significantly more lethal than a 2.5" permanent wound channel through the body. If you blow up an 8 lb piglet with a .223 or .300 win mag, is it any more dead with the .300 win mag?

Treat all indexes and calculations with a jaundiced eye. They are indicators of potential performance, but not determiners of actual performance. Far to often we see the calculations as determiners and that is not what they are.

Based on personal use and observation and plenty of venison eaten, my old 35 Remington killed deer as well or better than anything I’ve used since. Always an exit. Always a blood trail of significance. Always a short blood trail. I went from there to a 270 purely for a flatter trajectory and a longer useful range.

Again, some really great information in this thread, and the bottom line is the answer to lethality/damage depends on so many factors. That is why KE/ft lbs of force at the target is the only useful indicator of how likely a given round is to be lethal for the animal. Even these types of calculations totally DEPEND on proper bullet placement as we have all read or seen stories where a poorly placed bullet isn't lethal in a timeframe a hunter desires or needs.

While these thought processes are helpful to determine the proper gun for the job, the reality of it is any bullet (almost) can kill any animal (almost) if put in the correct place. The problem with hunting and wild situations is we rarely get to pick out our place and put the bullet exactly right there.

Excellent info on here guys. Excellent info!

One gun scribe that was high on energy of 1000 ft/lbs for deer, 1500 for elk and 2000 for big bears/moose wrote that type numbers a lot then went to Alaska and hunted and killed a bull moose with a 44 mag handgun which is below the 1000 he said you need for deer. Shot placement and bullet integrity with enough force to do enough damage to the vitals is what is needed. How much that is is purely speculation IMO. Like 603 pointed out.

I have killed a few critters with the same rifle/bullet and shot placement with widely varying degree of damage to the internals/lungs. Some dropped on the spot while others ran up to 100 yards those that dropped on the spot the contents of the chest could be poured out (liquified with chunks) while those that ran further just had much less damage. A few of those were either close enough I could see the breathing from the movement of the chest or moisture from the nostrils. Those that were shot during the exhale dropped on the spot almost like a CNS hit. KInda like a sucker punch hurts a lot more if you have exhaled before it than if the lungs are full of air. The lungs full of air do not transmit the expanding energy from the bullet as well as when they are deflated and more liquid.

Not in my opinion. I have pretty much moved exclusively to big bore airgun hunting. I have learned placement tops displacement every time. So the answer can't be found in a formula. The answer is on the range and in the knowledge you learn about the anatomy of what ever your hunting at the time.

Medium bores like the 338 Federal, 358 Win, 350 REM Mag, 9.3x62 and the like are often overlooked in today’s fascination with high BC long range mania. Those type rounds are often some of the best killers on large and small animals.

Great discussion and I can't disagree with anything.

I have gravitated toward the camp of reduced impact velocity, for reasons already mentioned. Something with a +3000 fps MV, and a 100 yard or less shot is not something I prefer. I just don't think it works as well, and why it doesn't work as well has already been lined out, it just doesn't dump the energy.

On the flip side, if you want to have effectiveness out to 500 yards, you do need to leave the starting gate with some speed. My 7 Rem Mag and 180's is the largest cartridge hunting setup I have. I'd rather not hit something close up with it (though I have) but it does a great job 200-500 yards. Bleed off some speed, so the bullet will expand and dump energy into the animal. That rifle hunts west, which is anything west of I-35 for me. Except whitetail.

10 months out of the year I'm running a 162 gr 7mm ELD-X at a mere 2640 fps MV. It just works, for everything I ask out of it.

Another classic is a .308 Win loaded with a 165 gr Sierra Game King spire point boat tail. Not very sexy, not a very high BC, but 200 yards to the muzzle it is a stone cold killer.