I have had spitfires completely crush upper leg bones and shoulder joints on the exit side but cant remember one of those that was a full pass through. Like mentioned above there is a fair bit of luck involved with how the point of the head hits bone and the resulting damage. Also cant recall a thunderhead not exiting no matter what bone was encountered going through. Keep in mind all of the deer I have killed with thunderheads were on arrow around 500 grains and running about 270 fps or slower. In most cases, much slower like 230. What my anecdotal evidence points to is that, for me, I would never shoot an arrow below 400 grains and with an arrow that light I would not shoot mechanicals and expect pass throughs consistently unless I cranked the draw weight up to 70+. The lighter the draw weight goes, the proportionally heavier the arrow needs to be for consistently good results. The other thing is fixed heads with thin tips, (bear razorheads, zwickey, stingers, etc), will curl or break if heavy bone is encountered. Maybe not every time but plenty. There is no perfect answer and I think the best a bowhunter can do for the least amount of headaches from tuning and lethality is to shoot the heaviest arrow they can keeping the fps in the 260-280 range and then shoot an appropriate head for the arrow setup, for compounds obviously. If a bow drawing 55# will push a 415 grain arrow 265, I would lean towards a fixed head like a vpa 3 blade, ironwill or daysix. If the bow drawing 70# will push a 500 grain arrow 270 fps then I would opt for something like a spitfire or grim reaper for bigger hole and expect pass throughs virtually 100% of the time outside of maybe a spine shot. Bow tune and arrow placement are more important that the actual arrow setup keeping in mind arrow setup in relation to tune. I think too may people get caught up in the heavy arrow or light arrow argument and fail to consider that an efficient bowhunting rig requires all components of the system to be evaluated for how they will be used together. It is an interdependent shooting system.