handy rule of thumb for energy increases

[mickb]

Thought I would share a basic formula for calculating the difference in kinetic energy when you either increase calibre whilst keeping the case capacity the same( like necking up a case) or increase case capacity whilst keeping calibre the same( like comparing 30-06 to 300 win mag case sizes).

Its not perfect but makes a rough estimate and was handy back when you were trying to figure out what safe max loads would be for new wildcats.

basically as follows.

Increasing Calibre
1.The increase in kinetic energy when necking up to a new calibre is equal to half the increase in cross sectional area. So if say necking up( or comparing) a 378 weatherby to a 460 weatherby you take the .165 sq in of the 460( .458 cal and using area of a circle formula) divide it by the .110 sq in of the 378( 375 cal) which gives about a 50% increase or 1.5x as much. Therefore the theoretical maximum kinetic energy increase of a 460 over a 378 is half that, or about 25%. if you take max loads for a 378 wby which are about 6200ftlbs, you end up with 7800ftlbs for the 460.

Increasing Case Capacity
2. where increasing case capacity the % increase in kinetic energy will equal half the increase in max water case capacity. So comparing a 458 lott with its 108 grains to a 460 wby you have the 460 with about 40% increase in case capacity which should mean a 20% increase in KE. Running the figures on max 458 lott loads of about 6400ftlbs this gives about 7700ftlbs for the 460 which is about right.

Now again, its not perfect, the length of the bullet intruding on powder capacity, the fact when you neck up you may also increase case capacity and max load data running at different pressures will all have an effect. Also some of the special high energy powders that have come out will prevent a comparison to normal powders.
'But generally where all else is equal its works out and if you compare max energy loads of 308, 30-06, 300 win mag and 300 weatherby you will see that half increase jump as the powder capacity increases.

'And as you neck up say a 375 wby to 416 rem to 458 lott, to 470 capstick you will see the same half cross sectional area increase in energy along the way.

You can also do one calculation after the other to compare increasing both case capacity and calibre. Say comparing differences between a 338 win mag and some new 40 cal wildcat you are considering based on a 404 jeffery case or something.

I got shown this in the pre-quickload days by a weatherby collector Mike Mcguire from Sydney. He was incidentally one of the first fellas to import 460's back in the late 1960's( well before my time)/ Is it still useful, well not as it used to be. But it was handy back in the day when trying to predict what the max velocities might be for unknown wildcat developments. If you can estimate kinetic energy you can work backwards to get the velocity for given bullet weights. In the early days of the internet we were also using this to predict maximum loads and velocities with the old nitro express rounds on modern actions. For ecample running figures for the cavernous 505 gibbs on a Cz550 for the first time by comparing to known velocities with the smaller 500 A-square case.

[disco stu]

I've long pondered this type of thing, from my bow days also. Wondering if I'm correct and it's just terminology, or if I'm actually wrong

Kinetic energy wouldn't change when you change projectile I thought, as kinetic energy all comes from the chemical energy in the powder being converted into kinetic. So is this based on increased capacity due to larger diameter mouth/increased capacity of powder, or is it more referring to down range momentum?

[bladeracer]

I've long pondered this type of thing, from my bow days also. Wondering if I'm correct and it's just terminology, or if I'm actually wrong

Kinetic energy wouldn't change when you change projectile I thought, as kinetic energy all comes from the chemical energy in the powder being converted into kinetic. So is this based on increased capacity due to larger diameter mouth/increased capacity of powder, or is it more referring to down range momentum?

Kinetic energy is the energy of a mass based on its velocity, nothing to do with powder only mass and speed.

[mickb]

Along with discos first comment, the basis is cartridges have a maximum kinetic energy across most bullet weights and it doesnt change a lot by subsituting different bullets. A 30-06 is a 3000ftlbs cartridge max normally for most bullet weights( if you check enough max load data), a 308win is about 2750ftlbs, a 375H&H is usually 4600ftlbs max and so on and so forth. So if you know that you can work the kinetic energy formula backwards to figure out a velocity for any bullet weight...

The reason necking up a calibre produces more energy generally is because it becomes a more efficient cylinder or piston.

And for whatever reason when you do so, or increase the max powder capacity a certain amount ,you get about half that increase in kinetic energy.... why would this be so, a question for an engineer. Doesnt make sense to me, but seems to work.

[bigpete]

Its more about the efficient transfer of energy into the target....

[Blr243]

I’m a huge fan of bullets staying inside expending all the energy into the vitals. I’m not looking for big exit holes and blood trails ... tome an exit is some of the energy wasted. What’s the point of trying to kill some of the dirt 50 yards further. It’s also so much safer if I can leave a property after a week and be 100 per cent confident that every Bullet I fired either blew up or stopped inside the game .. no stray lead flying around to cause grief. I’m believe humane hunting shots result from a combination of Bullet placement,Bullet construction and the energy Delivered correctly ... I’ll often flip a dead pig over to check the offside to ensure all is going to plan

[bladeracer]

Its more about the efficient transfer of energy into the target....

That's correct, a .30-06 might be making 2500ftlb of energy hitting the target at 100m at 2500fps, but if it exits the target still making 1500fps it's taking 900ftlb of that energy with it, which is all wasted. Use a bullet of the same mass but softer construction so that it doesn't exit and all that energy is dumped into the target.

I think a "normal" punch is around 100ftlb, a professional boxer can make more than 1000ftlb.

[disco stu]

I've long pondered this type of thing, from my bow days also. Wondering if I'm correct and it's just terminology, or if I'm actually wrong

Kinetic energy wouldn't change when you change projectile I thought, as kinetic energy all comes from the chemical energy in the powder being converted into kinetic. So is this based on increased capacity due to larger diameter mouth/increased capacity of powder, or is it more referring to down range momentum?

Kinetic energy is the energy of a mass based on its velocity, nothing to do with powder only mass and speed.

Well, yeah, but no (said in a friendly, non argumentative way). Projectile sitting of the counter has no kinetic energy as it has no velocity. It only gets velocity from the powder igniting behind it, so therefore energy, and all of that energy the projectile has comes from the powder. So it all has to do with the powder, which only has a certain amount of energy in it.

Kinetic energy=1/2 mass x velocity squared
Momentum= mass x velocity

They both rely on the mass and velocity. What I'm really asking is are we talking about momentum or kinetic energy? Which actually gets confusing as they're so close

What mickb said about kinetic energy not changing much between different projectiles in the same cartridge is one thing I'm kind of getting at, and asking at the same time. The powder only has a limited/set amount of chemical potential energy, that transforms into kinetic energy on firing and transfers to the projectile, sending it out with that much kinetic energy (ignoring friction in the barrel yada yada). If we don't change the powder amount have we actually changed kinetic energy, or is it only momentum?

I might be dragging this onto a different tangent

[animalpest]

If you change kinetic energy you change momentum. Both have velocity as a component so an increase in velocity increases both KE and M.

The problem with KE is that velocity is the primary change. That is, a small increase in velocity increases KE a lot, whereas increase mass and there is a lesser increase in KE.

In my view, KE is more important for small to medium game whereas momentum is more important for the big stuff.

[bladeracer]

Well, yeah, but no (said in a friendly, non argumentative way). Projectile sitting of the counter has no kinetic energy as it has no velocity. It only gets velocity from the powder igniting behind it, so therefore energy, and all of that energy the projectile has comes from the powder. So it all has to do with the powder, which only has a certain amount of energy in it.

Kinetic energy=1/2 mass x velocity squared
Momentum= mass x velocity

They both rely on the mass and velocity. What I'm really asking is are we talking about momentum or kinetic energy? Which actually gets confusing as they're so close

What mickb said about kinetic energy not changing much between different projectiles in the same cartridge is one thing I'm kind of getting at, and asking at the same time. The powder only has a limited/set amount of chemical potential energy, that transforms into kinetic energy on firing and transfers to the projectile, sending it out with that much kinetic energy (ignoring friction in the barrel yada yada). If we don't change the powder amount have we actually changed kinetic energy, or is it only momentum?

I might be dragging this onto a different tangent

If changing the powder changes the velocity then it will change the energy.
As we are not dealing in physics here I think we could consider momentum and kinetic energy to be similar enough that either will get across the same idea.
Momentum is potential energy, an object has momentum when it moves. But using the term momentum is likely to just confuse the person you're talking to as everything they will have learned will have discussed kinetic energy.
I tried to find a layman's explanation online but virtually every discussion turns to mathematical formulae which really helps nobody that doesn't already understand it. This was the best I found, and it uses firearms as an example.
https://www.real-world-physics-problems.com/difference-between-momentum-and-kinetic-energy.html

But, yes, for most of what we do with these things Mick's rule of thumb is fine. .30-06 for example uses bullets from 100gn to 240gn (it is possible to go further than these extremes a little but this covers virtually every variation that anybody has ever loaded I think), with velocities ranging from around 3550fps down to around 2200fps in the same rifle. Energy will vary from around 2600ftlb to around 2800ftlb (heaviest to lightest bullets). So even a very extreme example makes very little difference in the energy, about 10% over a very wide range of weights and velocities. For us as shooters and hunters, it makes far more difference that a 100gn bullet is likely to stop in, or on, the target whereas the 240gn bullet is likely to zip straight through, leaving very little energy in the target.

I discovered this when I started developing loads for Metallic Silhouette. I had assumed that a 200gn bullet had to have significantly more energy to dump into the steel than a 110gn bullet. But the combustion chamber is still the same size (possibly even smaller due a larger bullet) and can only burn a specific amount of fuel, both by physical constraints and for the integrity of the firearm's action. Because I was tossing the 110gn bullet about 45% faster than the 200 (I would have to check my log to get the exact figure but I think it was 1300fps and 900fps), it carries 15% more energy than the slower 200gn bullet, despite the heavier bullet having far more mass and momentum. Velocity is key to energy as it doubles with increases, increases in mass only increase energy by half.

[bladeracer]

If you change kinetic energy you change momentum. Both have velocity as a component so an increase in velocity increases both KE and M.

The problem with KE is that velocity is the primary change. That is, a small increase in velocity increases KE a lot, whereas increase mass and there is a lesser increase in KE.

In my view, KE is more important for small to medium game whereas momentum is more important for the big stuff.

Yes, think of momentum as "penetration" potential rather than energy.

[bigpete]

As a matter of interest,look up Taylor Knock Out values. Takes into account velocity, bullet weight,AND bullet diameter,although its quite haphazard

[mickb]

yeah Taylors KO was originally for comparing the effect of cartridges knocking elephants uncouncious for headshots that missed the brain. And it was pretty much debunked. He spoke about estimates of a 600 nitro knocking elephants out for 40 min, 577 nitro about a half hour down to 5 min for 470 nitro. When the safari industry kicked off again in the 80's and 90's no one saw any of these knockouts really happening. These days people use his formula for everything from shooting bear to self defence pistols lol. Might as well, because it certainly doesnt work for knocking out elephants.

As to my original post it wasnt about killing predictions btw, its just a velocity predictor. If you have a case and calibre of a certain size and you increase those dimensions, you can roughly estimate how fast bullets will go in the new case/calibre.