Robert, you made my point for me. NONE of those recovered heads were from inside the chest cavity - because their penetration stopped before they got that far. Yes, some were stopped on shoulders or spines, but some were also stuck into entrance side ribs. Judging from appearance of the imbedded head in relation to the bones involved, I would say most were on angling impact. Was the angle selected by the shooter, or did the deer jump? Was the arrow deflected by a bit of grass or leaves? Doesn't matter. The outcome was the same; a non lethal hit on a shot where the angle of impact indicates it could have been a killing hit ... if the arrow had penetrated enough to reach the chest cavity.
The others? All they show is that bad hits are going to happen, no matter what. That's even more reason to use an arrow setup that will perform even on the worst of hits. It might not turn every single bad hit into a clean and humane kill, but it will do so for more of them than a lesser arrow setup will.
It is never wise to hunt with a setup that REQUIRES the shooter to make a perfect hit in order for his shot to be lethal. Animals are not practice targets. When we shoot them, our purpose is to kill them. Unlike a poor hit on the range, a bad shot on an animal ends up with more than just a poor score; and it makes no difference whatsoever WHY the hit was not ‘dead center’. Your arrow should be capable of killing on ANY hit that’s HIGHLY LIKELY to happen, not just on the ones you are fortunate enough to 'hit perfectly'.
Among the bowhunters we guided in Africa (none of which can be called bad shots, in fact, most were WAY above average – at least on targets); on hunts were multiple animal taken were the norm; I can not think of A SINGLE ONE who did not make at least one poor hit ... but CAN think of a few who didn't make a single perfect hit, and some who didn't even make a single 'well-placed' hit. Virtually every ‘non-recovered’ animal was a result of poor arrow penetration. That’s not based on just the appearance of the penetration at the time of the hit. Many of these animals subsequently required putting down with a rifle; either after a long follow-up, or later, by the game scouts. I got to actually examine many of those non-lethal hits.
There's no penalty when you err on the side of ‘too much’; there’s definitely is a penalty when it’s “too little”. The ‘I can hit more accurately at longer ranges with a lighter arrow’ or 'using a heavier arrow will give me more poor hits at longer range' arguments shouldn’t even be raised. Bowhunting is not about hitting at longer ranges; it’s about the challenge of pitting your hunting skills against the animal, beating them at their own game. Well, unless you’re one who bowhunts only so you can kill an extra deer or two.
If one is not bowhunting for the joy and challange of bowhunting, or doesn't have the desire, time, patientce or skill to learn to get close to game before shooting – please, please, please - just use a rifle.
Ed