Quoted from Dr. Ashby by others:
"As I point out in the article, not all momentum is equal! Momentum gained from increasing arrow mass [ie. heavier arrow] results in more penetration than momentum gained through increased arrow velocity [ie. speed]. The data here is clear cut [ie. more momentum with a heavier arrow]. The reason is because the increased tissue resistance encountered by the faster arrow (assuming both arrows have equal impact momentum) reduces the time over which the available momentum is disipated in the tissues: the lighter arrow will have a lower Impulse of Force - regardless of the distance of the shot."
The tricky part here is where he says "....(assuming both arrows have equal impact momentum)...." Because, in the first sentence he states "....not all momentum is equal!"
I understand what you say Len, and how you read it, but it still makes sense to me. I think that he wanted to show that arrows with equal momentum at the point of impact, one being light and fast, one being heavy and slower will not have equal penentration. Therefore equal momentum at the point of impact does not equal penetration because of increased tissue resistance for the light, fast arrow.
I wish we could have gotten him over here, to clear things up, but he is now doing some research in New Zealand.