1. If it's based on velocity, that doesn't make any sense, since high velocity projectiles often fail to incapacitate as can be seen with 5.56 quite frequently.
It is based on velocity, but as I mentioned quite a few posts ago, that you need more frontal area to cause significant hydrostatic shock. A 5.56 has trouble incapacitating because they use FMJ's in the military. The earlier 5.56 bullets did not yaw as early, and because of that mostly penciled through. .224 has a small frontal area it's a skinny long bullet. When it yaws the frontal area is increased, because the 'side' of the bullet is now the 'front'
Expanding hunting ammo and an early yawing 5.56 will be much more reliable at incapacitating.
There are people who hunt deer with .223/5.56 and can put them down in 1 shot with good placement to the chest. The red jelly that used to be healthy tissue, that gets left behind in the wound cavity is pretty decent evidence of hydrostatic shock.
5.56 FMJ has velocity but frontal area is pretty minuscule when it's going straight. OTOH some .223 bullets are over an inch long. When it starts going sideways, that's a pretty dramatic increase in frontal area.
2. If it's based on energy transfer then it goes hand-in-hand with energy-transfer/radial temporary cavity-this would mean that the projectile still needs to yaw or upset in some way before it can cause serious wounds.
Exactly what I stated a few posts ago. Like I said before, FMJ's even at high velocity, don't do much damage if they don't yaw. They pretty much pencil through. And there's plenty of evidence in this thread that supports that. All the FMJ wound profiles look like pistol wounds until that bullet starts to yaw, where the cavity opens up. And that's demonstrated with EVERY FMJ bullet from every cartridge, in this thread.
There are instances where you could get a (non yawing) FMJ to have more devastating terminal effects, but it pretty much involves increasing the frontal area with a larger caliber bullet, and flatter, less streamlined front. A bullet with a flat front would be more likely to have serious tissue devastation without yawing or expansion, but performance through the air would suffer significantly, as it would reduce the bullet's BC, since you're intentionally giving it a less streamlined shape, in air, and fluid. Because of it's less streamlined shape, in fluid, it's going to need to rapidly push the fluid out of the way as it travels through it, which is going to have more violent effects. Still it's not going to be anywhere as devastating as something likely a .30 cal hunting bullet at 2800fps, which expands from .308 to about .60-70 caliber once it's in the target.
You need both rapid energy transfer and velocity. Rapid energy transfer pretty much requires a large frontal area. Military can't just move up to a big bore .50 cal for everything because there are many other considerations than damage, when choosing a cartridge for military applications. Recoil, resources/materials, weight, wind resistance. Bigger bore has more of all of those, which are negatives especially when bullet materials are expensive and hard to find during war time.
