As already mentioned, the reason for the higher KE number due to speed, having a greater role in the equation we use to calculate the amount of energy. That's all.
For example, the trend at one time was to shoot very light 185 gr bullets in 45ACP to get velocity and energy numbers elevated. That looked good on paper, but those bullets were very short in relation to diameter. Plus, at the speed they impacted they expanded violently. Those factors resulted in poor penetration.
A 180 gr bullet fired from a 40 S&W at about the same speed is much longer in relation to diameter and results in better penetration.
While this accurately describes the results, the reasoning is incorrect. It is not the
length of the bullet that is the prime factor in penetration, it is the
width. And MASS and SPEED, all combined.
Length results from the restriction of bore size on the mass of the projectile. Given equal mass (weight) and using the same material, the smaller diameter bullet MUST be longer. There is no getting around that. Length is the result of given weight and bore size.
It is not the length that determines how well something penetrates, it is the width, specifically the size and shape. Pointed things poke better than blunt ones. Narrow things poke better than fat ones. When the available force is the same the smaller diameter object will penetrate easier, resulting in deeper penetration.
As a simple example of the principle, say you step on a nail, the force of your foot coming down is the constant. Same amount of KE. Step on the point of the nail, it punctures your shoe and likely your foot. Step on the head of the nail (much wider compared to the point) and it usually doesn't puncture your shoe. Length of the nail doesn't matter. Width of the contact area, does.
Those 185gr .45 bullets that were driven fast and expanded too much and didn't penetrate well didn't underpenetrate because they were short, they under penetrated because bullet construction was inadequate for the impact speed, and they expanded too much (making them WIDER) and more of the available energy was needed to push them through, so they didn't go as deep.
Now, on to KE. Some folks will probably put me in the group that says KE doesn't matter, but I say the number you get on paper using the standard formula doesn't matter the way most people seem to think it does.
First, as you've seen, the math is heavily slanted to give velocity the most "weight" in the formula.
Second, the amount of energy needed to do a specific job is what ever it takes to get the bullet where it needs to go to do the job it needs to do. And THAT will vary considerably. More than needed is rarely a problem, less than enough can be a very serious problem. Since we can't know how much will be enough we strongly tend to use as much as we can.
And that is what the calculated KE numbers are for. For a relatively fair comparison between the very different calibers, and speeds, based on a common standard. If round A calculates to 700 and round B to 450, that shows clearly A is significantly more than B, on paper.
This does NOT translate DIRECTLY to performance in the field. There is a relationship, but it is not mathematically linear or constant like a formula. There is no "dead+" or "stopped ++",
Once the KE level is above the minimum needed to physically perform the task, the paper KE number is not relevant, it is other factors in combination that determine how well the shot "works".
Another example, (using rifle rounds to illustrate the extreme difference possible)
You can load a .22-250 and a .45-70 to an IDENTICAL amount of KE. EXACTLY the same amount of KE using the standard formula. Very small bullet, moving very, very fast, vs. very large bullet moving much, much slower, giving exactly the same results using the KE formula. SO, based on KE alone, they should be the same in the field, right?
They aren't. And that's because KE isn't alone. It's only one factor among many, and those other factors have a more dominant effect in the real world performance than KE alone does.
.22-250 vs. .45-70 with equal amount of KE in both. Which one of those would you pick to stop an angry large animal?? On paper using KE alone, there should be no difference, but in the real world there is a huge difference and that is because of factors other than the calculated KE number.
I know which one I'd pick, by choice, and its not the .22 caliber one.
