Both posts, excellent information,good work and accurate. I only went into the two types of energy measurements. Regarding the harpoon, yes. A long solid projectile with the sectional density of a crowbar would plunge through.water, but take that same mass and turn it into a cannonball,and every characteristic of the test changes, because the projectile was changed.
When compound bows were first available here, a guy set up shop and put a display in his shop. Two large, loosely filled sandbags. One had an arrow punched through it and the other had a little hole with a black ring marked around it. He claimed to have shot the arrow through the bag, and used a "deer rifle" on the other. The bullet supposedly didn't make it through. Myself, I didn't really believe his story, but the principle is sound. I could push an arrow through the bag, even. The bullet will either disintegrate or expend all of its energy tossing the sand around.
The point made about energy on target is so important, as Brandon pointed out. Ever wondered just how much chemical potential energy is present in ten grains of powder? Very little. Then, half of that energy is expended in recoil. Take a hollow point bullet that you have tested, set a steel ball on top of a new one, and hammer on it until you have deformed it about the same amount as the fired bullets. You see, only a fraction of that ten grains of powder is actually used on the target. Between heat loss, loss through mechanics, loss through friction with the air and so forth, and loss spent on popping open that jacket, all you have left is a little bit of power. A typical person with a sharp half inch rod could punch though a man's torso with no difficulty.
Pilpen, you asked at some point whether two rounds, one heavy and another slow, would release the same amount of energy on the target.
The truth is that if the have equal momentum, with no other complications that will absorb energy,they will both transfer the same amount of energy as calculated by momentum. When measured by kinetic energy, the lighter, faster bullet will have the same amount of momentum, but when measured as kinetic energy, the number will be higher for the fast bullet.
As we've discussed, high velocity is a destructive force. An ultra high velocity light round equal in momentum to a low velocity heavy round will create massive destruction of tissues. Blood will be turned into mist, bone will be ground to sand, exit and entrance wounds will be large tears. Heavy and slow tend to just push their way through.
Experiment a little. Take water balloons. See what a standard non-hollow point pistol bullet does. Then try the smallest rifle round you can get. Ultra velocity rifle rounds don't splash.
