Marrakia is correct, in that we have to compare "apples to apples"If we take the "TKO" of a 30-06 with a 168 grain bullet it comes out to 19.58
BUT if you had a US HI 1945 black tip armor piercing round and a good soft point of the same weight, the formula doesn't work, because although the energy of the 2 bullets is the same, the soft point dumps it's energy in a shorter and wider wound in an animal, and therefore kills and disables far better then the AP round.

We need to compare bullet type to bullet type and bullet placement to bullet placement to have a true scale to evaluate. A 7X57 with a FMJ through an elephants brain is much faster to kill then a 700 Nitro through it’s lungs. But that proves nothing

However, the other side of the coin is this;
In the case of both the 16 bore muzzleloader and the 458 Winchester, it must be remembered that the 45 cal bullet will expand to about the same size as the 66 cal ball, and the wounds are going to be about the same. It's the WOUND that kills and disables, not the gun, not the shell and not the bullet. The bullet only makes the wound, but the wound is what results in death or incapacitation.

Don't sell these old fashioned front stuffers short. until you have hunting with them, you would not think they would work as well as they do. Believe me, they kill WAY better then the Paper energy" would have you believe, just because they make very big and deep holes in things. Elephant is the one animal that they have had some trouble with historically, but they still did a grand job for years. On anything smaller then elephant, they do as well and many times better than most modern cartridges. Only the very largest modern shells do as well as a good 8 bore, and I doubt there are any that do better then a 4 bore.

The 2 bore is somewhat of an oddity in that to get it’s velocity up to where it penetrates well on elephant, it was known to break bone with it’s recoil. But if a man could carry one and stand the recoil there were truly awesome. Even with only 24-30 inches of penetration on elephants, it was something of a “cannon” just because of the terrible trauma it causes to what ever it hits.

I am learning about the 2 bores right now. I have gone over and over all I can find about the old ones and I was surprised to learn most were not too much heavier then the 4 bores. Now that I am making one, I see why
To get the weight up to 30 pounds you have to make the barrel so large on it’s outside dimensions that you can’t make a stock and grip small enough to get a n average man’s hand around. I am adding weight everywhere I can to get the one I am making now up in weight, but I am still coming up short of my targeted weight.

I also find that the powder charges listed in the old books and writings were about the same as the 4 bore charges. Again, I know why now. If you were to double the powder charge over a 4 bore as you double the ball weight, I think the recoil would do Permanente damage to the shooter in a 22-25 pound gun, and as I said, to make the gun at 30+ pounds, you can’t hold on to it because you can’t close your hand around it, so it would jump out of your grip if they were that big.

In these big rifles the recoil is something different then even a 505 Gibbs When I shot the Gibbs the recoil was heavy, but not horrible. The 4 bore was W AY more, but when I had the 17.5 pound rifle moving, I could not seem to stop it. It’s like the kick was hard, sharp and LASTING. unlike a 505, the recoil didn’t seem to stop with the noise. It’s like it keeps on coming for a time after the shot’s fired and the smoke is clearing.
I can’t even imagine how the recoil would be to have that kind of velocity with a 2 bore. (1650 - 1800 FPS)

Even 300 grains of powder with a 2 bore is frightening to me.