You're looking for knockdown power to mean something is actually going to be knocked down by the initial hit of the ... whatever, and I don't know why. You're forcing a specific result when all kinds of varying possibilities and results are out there.
But here, picture all of this like this.
Imagine I had an F250, I put a 20' long 1/100" diameter spike off the front of it, you stand and I hit you at 100mph, but here's the thing, the truck never hits you, just the spike, what happens?
Now, take away the spike and I hit you with the truck at 100mph, see the difference?
Basically the same weight and same speed, what changed, area of impact. Bullets are tiny areas of impact, that will create a very different effect. That's one point to be addressed here.
Another, we use truck again, truck good, you know truck.
Truck has suspension, yes, yes, good.
Truck have rubber tire with air in it, yes, yes, good.
Both components absorb and tire area on road disperses the energy of your 7,000lb truck bouncing. Produces comfortable ride, yes, yes, well, relatively.
Imagine for a moment you have train tracks ahead, now these are those train tracks that are like a ramp and you've been dying to try and jump this, so you do.
What happens with proper suspension, suspension components, and wheels? You land, drive off, street isn't destroyed, you didn't get whiplash, didn't break your back or neck.
The energy of that 7,000lb truck traveling 100mph is still there, it's just absorbed and dispersed by the suspension and components, and tires, and dispersed over the area of the tire contacting the ground. Road isn't destroyed and neither are you.
Now, imagine you do that same jump, but we replace your tires with steel blocks the size of tire tread patch and you have no suspension, now imagine the truck is magic and doesn't fall apart so all of that is transferred to the ground. Potholes, smashed up pavement for the road.
You can imagine being in that truck as it slams and smashes down, the abuse evident, may now slam your head on the roof, break your neck, back, etc... Same energy, same contact area, very different result.
Now imagine that but with smaller contact area, or bigger, imagine skis of steel that are 3' wide and 100' long, the energy force being dispersed in such a large way the road may come out unscathed, though you in the truck will still feel abused.
Moving on:
So, using a steel plate as an idea isn't bad, as long as it doesn't give, as in, no dimple or bullet passing through. Energy will transfer in a pretty blunt way, picture silhouette targets that tip over... well, shoot one of those little 22 ones with your 458 lott, doesn't just tip does it...
And why using a human with a bullet proof vest won't work for your desired result, because behind that bullet proof vest is suspension, in the form of skin, a skeletal structure, liquid, organs etc... Not to mention the give of the vest itself.
The energy is absorbed in a different way, so the person or animal doesn't just go flying through the air. Even something as simple as picture someone pushing you when you're standing with your legs way apart vs together, same energy used, vastly different results.
Also, penetration of bullet through object doesn't dump all the energy, there is obviously that to consider as well.
So, just because the thing doesn't go flying, doesn't mean energy isn't a thing.
And most here know energy is big, stand tall with your feet together and fire a 500 Jeff that has a steel buttplate.
You'll likely see the animal knocked over, the animal being you. Affect would be much worse if your body weren't allowed to rotate, allowed to compress etc... to absorb some of that energy.
Does energy ultimately matter though, in killing an animal, sure, more disruption is more disruption.
Certainly it can be a HUGE component in a stop scenario. We see that in some of Mark Sullivan's videos, his first charge with the 500 NE, bullet hits between eyes misses brain just low, buffalo doesn't even react.
In later videos we see him use a 600 NE and a 577 NE, several instances where he misses brain but the animal is still incapacitated.
Like getting hit by Tyson vs your 12 year old daughter, Tyson puts you out, but your daughter is sleeping with your best friend.
