The "Best" Measure of Lethality
(Continued)

Hrdlicka also provides a “rough guide to hunting effectiveness based on kinetic energy from an archery chart:
 
    < 24 ft lbs.            Small game
    25 -41 ft lbs.         Medium game (deer)
    42 -65 ft lbs.         Large game (elk, bare etc)
    66 ft lbs.               Toughest Game (cape buffalo, grizzly, etc)
 
Kinetic Energy
While kinetic energy is very good at summarizing the key parameters of a weapon, he points out that a baseball has a lot of kinetic energy and yet it would almost never be lethal. 
 
Momentum
Hrdlicka argues that momentum is important, because the momentum is conserved while the projectile is trying to apply the necessary sheer strength to pierce mammal flesh.  The spear and the atlatl because of the weight of the missile can have equal or greater momentum provided they are used at close range (30 to 50 yards).   >> See Newton on Penetration >>
 
Sectional Density
Finally he notes that “a ping pong ball thrown at a pop can will bounce off.  A BB will go right through”.  The difference being sectional density, and the spear, atlatl and arrow actually have a better sectional density than firearms.  Their heavy projectile’s momentum is better conserved as the projectile pass through flesh (1 gram/cm3) hide and bone (2 gram/cm3). 
 
  His final conclusion is:

    “All the weapons listed have been used for killing, and all have proven themselves effective.  Altalatl darts do not have as much kinetic energy or momentum as firearms, but have a better sectional density.  They are higher in all three factors than arrows, and arrows are known to be a very effective weapon.”

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