For what it's worth, here in Arizona Center of Mass shots (on multiple assailents) are getting to be less effective, as many of the incidents involve multiple assailents wearing body armor, armed with rifles (AR/AK) and also dressed in tactical POLICE clothing, and well trained (scary shit!). More and more Deputies here are training to engage multiple targets, with Head and Pelvic shots...a much more difficult proposition!
But, many are also doing it more with an AR platform when the time comes to respond to a hot call. The pistol is only a last resort if caught by surprise, mostly prevented by situational awareness. And, I am speaking from inexperience, but I still believe I would still prefer a 12ga. w/00 buckshot for those close "pistol distance" house distance type encounters (or 7.62NATO to punch thru :), but I understand overpenetration...but then, going home at the end of the shift, etc).
I have to train for close pistol encounters (currently with a .38sp), because that's all I typically carry...and would most likely be a close in, self defense situation against a crackhead (or three) without a vest.
But, I still practice at less<7yd-to-contact distances on 3x5 cards at headshot height. Just in case.
Training for Most likely scenario...(and esp. with a .38sp headshots more effective).
5shot till reload...every one must count!