Interesting FBI report on caliber choice and bullet 'knockdown power':

http://www.firearmstactical.com/pdf/fbi-hwfe.pdf


Excerpt:

Goddard amply proves the fallacy of "knock-down power" by calculating the heights (and
resultant velocities) from which a one pound weight and a ten pound weight must be dropped to equal the
momentum of 9mm and .45ACP projectiles at muzzle velocities, respectively. The results are revealing.
In order to equal the impact of a 9mm bullet at its muzzle velocity, a one pound weight must be dropped
from a height of 5.96 feet, achieving a velocity of 19.6 fps. To equal the impact of a .45ACP bullet, the
one pound weight needs a velocity of 27.1 fps and must be dropped from a height of 11.4 feet.


And the final recommendation after 16 pages:

Kinetic energy does not wound. Temporary cavity does not wound. The much discussed "shock"
of bullet impact is a fable and "knock down" power is a myth. The critical element is penetration. The
bullet must pass through the large, blood bearing organs and be of sufficient diameter to promote rapid
bleeding. Penetration less than 12 inches is too little, and, in the words of two of the participants in the
1987 Wound Ballistics Workshop, "too little penetration will get you killed." Given desirable and
reliable penetration, the only way to increase bullet effectiveness is to increase the severity of the wound
by increasing the size of hole made by the bullet. Any bullet which will not penetrate through vital organs
from less than optimal angles is not acceptable. Of those that will penetrate, the edge is always with the
bigger bullet.