Here is my "physics" take on it:
Beman ICS Hunter
400
8.4 GPR
0.293" dia
0.92 sq. in/in surface area
Easton Axis FMJ
400
10.2 GPR
.262" dia
0.82 sq. in/in surface area
Based upon these two arrows there are two things that pop out (assuming arrow speed is identical).
First, the FMJ has 11% less surface area than the ICS. Regardless of the material the arrows are shot through, the FMJ would have 11% less friction acting upon it to slow it down. However, I would tend to agree with Hoss that with the lubrication of the body fluids and fats in an animal, that this frictional number would be relatively small and would not likely slow the ICS enough to not get a pass through.
Second, the FMJ weighs 21.4% more than the ICS. This will lead to a 21.4% increase in kinetic energy, which we all know leads to better penetration. I would guess that this is the main reason for better penetration with a FMJ arrow.
However, if the two arrows being compared were the same weight then that would throw out the kinetic energy factor. That leaves only one thing that I can think of, shaft flexibility.
While a cylindrical object, such as an arrow shaft, usually gets stronger as the diameter increases, it is possible that the narrower shaft has a thicker sidewall dimension. If this is the case, the moment of inertia of the narrower shaft might be more than that of the thicker shaft. A larger moment of inertia means less deflection of the shaft when shot and upon impact.
A simple way to think about this is imagine trying to push a sharpened pencil from the eraser end through a taught piece of paper. Goes through pretty easy, right. Now take a piece of sharpened uncooked spaghetti and try and push it through the paper from the back end. I would imagine that spaghetti stick would bend quite a bit before penetrating the paper or breaking. While this might not be the best analogy, I think it at least provides an image for understanding how shaft flexibility may play a role in it.
I would like to note that I have shot ICS Hunters for 10+ years now. My bow is the same age as the arrows and is only shooting about 250 FPS. In all the deer I have shot with it, I can only remember 3 that didn't pass through. 1 I spined, 1 I buried it in his front shoulder (didn't find, but was shot by gun hunters on our farm 1 month later with the arrow still there), and 1 that I think buried in his offside shoulder but never found to confirm. [/quote
Awesome thanks