[Deathblob]

Yeah, with a sling bullet, it has its own momentum, and that's it.

With long bows, at the point of penetration, behind the head you've got a great big long shaft pushing the head forward. I suspect the depth of penetration is proportional to the momentum of the shaft.

There are other factors affecting penetration too, like the shape of the head.

Also, the rigidity of shaft affects penetration. Comparing crossbows to long bows, the shaft is not nearly as long, but it is more rigid,