There is no question in my mind that the ongoing discussions of minimum caliber will go on forever.
• On the small caliber extreme side: A well placed .22 rim fire, sometimes lucky or unlucky, shot has stopped individuals. In fact there is some truth in the fact that the .22 is sometimes called "the assassins choice", but let's get real the pistol round, any round, is notoriously under powered if forgiving, instant stopping power is desired. That's simply the nature of short barreled, bullet velocity generation.
• On the realistic extreme side: You LE types are certainly aware of the super staying power demonstrated by "crack" driven perps. and the many case histories of ineffective large caliber hits on these individuals. It is not unusual to hear of mind altered perpetrators having sustained, for long periods of time, multi-hits with calibers such as .357, .40 .45 ACP, and sometimes .44 mag.. I do not have any data on .50 cal hand gun incidents as there are very few, if any authorized users of this caliber in the business.
Although I love hand guns, I suppose I have to agree with our Long Gun Instructor Officer Chris Mayer of Anaheim P.D., when he states "As far as I am concerned, the hand gun is only of use to fight your way back to your rifle that you so stupidly left behind."
I continue to stay with Jeff Cooper and will always stress large caliber - as large as practically controlled for the hand gun. For me that is the .45 ACP, although the .40 is not a bad compromise. The hotter 10 mm has a good lethality profile but I believe that the recoil resulting makes it a difficult round to control for most folks, especially in the sort barrel versions so typical of back up weapons.
Again remember the basis of the modern technique of the combat hand gun is the balance of Speed / Accuracy / Power.