What about when it runs into something solid? You tell me.

Fortunately, I haven't had to bust through that many bones in my hunting, but I have done it... even using wood arrows. I seriously doubt that the arrows I've used since switching to aluminum in 1979 (2216, 2219, 2419 and now, 2315) have flexed nearly as much as a wimply 2213 would on a bone hit. But then, I've never had a high speed camera handy when I've taken a shot either.

You must try to understand that a stiff spined arrow would flex less on a bone hit than a softer spine arrow. Since a lot of people have tried to use light, marginally spined (in the case of 3D, probably grossly underspined) aluminums for more speed rather than just going carbon, it generally seems that everyone is comparing carbon to 2213's in both penetration and durability, and that's REALLY comparing apples to grapes.

By the way, I never said an aluminum would give away a 'great deal' of energy in flexion. Does it give up any more energy by flexing than a light arrow gives up to wind resistance before it gets to the animal? Who knows...