Here is an article I just saw on Lew Rockwell discussing the 6.8 SPC as a good defense round. The 6.8 information is the last half of the article. The chart gives a good overview of the available barrel specifications.
http://www.lewrockwell.com/calderwood/calderwood29.1.html
http://www.lewrockwell.com/calderwood/calderwood29.1.html