"Though momentum is a 100% perfect and accurate measurement of forward force, momentum is not a 100% perfect indicator of penetration potential. A momentum factor that is derived more from arrow weight than arrow speed will penetrate better. Though they both may hit with equal force, the heavier and slower arrow will penetrate better than the lighter and faster one for two reasons.  First, an arrow penetrates an animal, the arrow speed diminishes, but the arrow weight doesn‘t. That said, arrow weight has more of a bearing on actual penetration potential than arrow speed does. The second reason is that force of drag increases with more arrow speed.

This speed vs. weight phenomena is another reason that the kinetic energy equation is not accurate in predicting penetration. If you’ll remember, the velocity factor (speed) in the KE formula is squared, or doubled (V2), so the extra emphasis on speed further degrades its accuracy as a predictor of penetration. Since the momentum formula doesn’t square the speed factor, this effect is more subtle, but does still exist.

So, neither Momentum nor Kinetic energy is a 100% perfect predictor of penetration; momentum is just simply the more accurate of the two.

Here is an example of two similar momentum specifications but different penetration potentials.

Bowtech Insanity CPXL @68 lbs. compound shooting a 500 grain arrow @ 278fps = .617 slug/ft. of momentum

Bowtech Insanity CPXL @78 lbs. compound shooting a 440 grain arrow @ 331fps = .619 slug/ft. of momentum

Both setups are nearly identical in momentum, but the second has to pull an extra 10 pounds of draw weight to get that same momentum! Not only that, the penetration potential of the 78lbs. setup will be inferior to the lighter bow, because as we stated earlier, speed will drop off during penetration, and arrow weight won’t. Compound this with the added resistance a faster moving arrow has to overcome (Force of Drag), and it becomes obvious that even with identical momentum specifications, the penetration will not be the same."

An excerpt from the book "Can't Lose" by Jeremy Johnson