[QUOTE=Stranger On A Train]
I was just going to say the same thing. The FBI’s rating system for the stopping potential of calibers emphasizes penetration. Note, however, that you don’t want excess penetration, as a bullet that penetrates through and through both fails to apply maximum energy to the target and poses a threat to bystanders.
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The FBI eventually concluded that the 10mm Auto round offered the best of both worlds–penetration and expansion–but discovered the pragmatic reality that the high recoil and large size of handguns chambered for the 10mm Auto were excessive for even average-sized agents, much less shooters with small hands or weak grip strength. (Anyone who has fired one of these monsters will know what I’m talking about.)
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A Special Agent in the Firearms Training Unit wrote up a set of notes on the decision to choose 10mm Auto (FBI load, which eventually cased down to .40S&W - full load is indeed a monster of a gun), which looks to be a fairly good reference. Choice quotes:
Regarding penetration:
“Minimum 12 inches penetration, & 18 inches is better.”
Third, as our testing shows, even the most frangible bullets designed specifically for shallow penetration will plug up when striking wood or wallboard and then penetrate like full metal jacketed ammunition. We are aware of successful legal actions where an innocent party has been struck by a shot passing through a wall, but as we have proven, ALL of them will do that.
Regarding caliber:
“Given desired penetration, only way to increase effectiveness is to make hole bigger.”
Regarding shot placement:
“If shot placement was all that mattered, we could arm all Agents with .22’s.”
A good example is killing a 400 pound pig with a .22, something commonly done on the farm. If the shot placement is exactly right, the pig is instantly killed. If it is off less than an inch, the pig goes wild and the process of killing it becomes rather lengthy and involved, whereas a larger caliber would succeed with a larger margin of miss than an inch. (Larger calibers are not used because they ruin too much of the pig - a consideration that does not come into play in a shooting incident - and besides which, nobody is going to die if the pig is not killed instantly anyway. In shootings, just the opposite is true.