ORIGINAL: ACPOSSETIM
Spitfire baby! They are a tough, solid, simple head that cuts a massive hole, leaves great blood trails, doesn't need o-rings or rubberbands to keep them closed, fly great, and only require about 45 #'s of K.E. to open.
here is a link of some slo mo test footage it shows a spitfire going through a cow femur and opening on a water balloon.
http://youtube.com/watch?v=Byvk_D35aOQ
Also they make a practice blade that replaces your hunting blades. I shoot them into my block with zero damage and the same point of impact.
AND,require a boatload of energy to get a passthrough.
Over the years on this forum and watching the "pros" on tv,I can say without a doubt that these head require much more energy than most mechanicals to achieve a full passthrough.
45# of energy to open is a freaking ton of energy to open a head.The heads I have used Rocket Steelheadsfor years and theyoutpenetrate most 3 blade fixed heads.