RE: poor penetration
What we sometimes think is poor penetration ......due to only seeing a portion of our arrow in the animal as they flee is......actually the arrow having backed back out to that position after hitting something inside. Happened to me on my first deer ever.
If you'd asked me how much penetration I got on that deer....I'd have told you 1/2 my arrow. In reality.....I had an exit wound on the off-side....but the arrow struck the off-side leg bone and backed out to that point.
There's NOTHING inside a deer's chest cavity to warrant stopping an arrow. IMO.....it's hitting something on the other side (in most instances) and backing out to where we see it as they flee. I'm not saying it can't happen (penetration of only a few inches).....but I'm betting my explanation is the norm and not the exception.