Quoted: The sand readily absorbs all the energy in the bullet. It loses interia and momentum almost immediately. An arrow with its long length will actually quite easily go through a sand bag. And that s because there are parts of the arrow shaft still with energy enough to drive it through.
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I don't think that is quite right.
A rifle bullet has much more energy than an arrow.
If you could keep the bullet traveling "pointy end first", it would penetrate the sandbag.
But since it tumbles, and the profile has much more resistence in the sand, it is slowed much quicker.
Sand backs itself up. I forget what the scientific term is.
It is like silly puddy. Soft and strechy if moved slowly.
More firm if struck suddenly.
You can easily push a bullet through the sand.
But at higher velocities, the sand compresses and is backed by what is behind it.
The force is spread out to an area MUCH larger than that of the bullet.
Keeping the projectile heading straight into the sand, it would push the sand out of the
way, virtually aerodynamically, instead of pushing it forward.