I'm a law enforcement firearms instructor and over the last 23 years I have participated in ammunition evaluations for my former agency of 4,000 officers, the Federal Law Enforcement Training Center and a combined municipal, state and FBI evaluation of pistol and rifle ammunition performance through light, intermediate and heavy barriers. Anyone interested in the mechanics of handgun wounding needs to start with this publication from the FBI:
fbi handgun wounding factors and effectiveness : Free Download & Streaming : Internet Archive
and here:
http://www.dlgunsmithing.com/uploads/4/5/8/2/45825609/wound_ballistics_2013_gary_roberts.pdf
and here for rifles:
http://www.dtic.mil/ndia/2008/Intl/Roberts.pdf
The next place to visit is this forum post by Commander Gary Roberts, USN (ret). Roberts is a trauma surgeon specializing in ballistic wounding and surgery of the head, neck and maxiliofacia. I have worked with Roberts and believe me when I tell you that he knows more about ballistic wounding than anyone in North America.
Self Defense and Duty Loads...ballistics info by DocGKR
Handguns, in short, offer poor incapacitation potential and are not optimal in most scenarios involving threats two-legged or four. However, since we cannot pack rifles everywhere 24/7/365, we must squeeze every advantage out of a handgun that we can. Accuracy is key, penetration is key, expansion is a bonus and generally not as critical as the former two points. Sectional density is paramount in bullets:
Sectional density - Wikipedia
If you read these links, you'll have a basic understanding of ballistic wounding.