Stopping power is a myth. There are rounds that generate more force and all that, but the "pick 'em up off their feet, and kill 'em dead" round doesn't exist. All that educated physics stuff Sir Issac Newton spoke about exists and if it does that to the bad guy, it does that to you...
And legally speaking, you're responsible for EVERY shot. I think the point you're making is if you're in a defensive situation do you want to have to shoot a fewer number of times vs a lot. Which the one variable can't account for is human behavior and condition. A 124 grain 9mm HST hits a wooden 2x4 with the same force as a human torso (within a +/- relationship). But the human's fight or flight response dictates different things as to how THEY respond to that. 9mm expands but .45 doesn't shrink and all that jazz...but the common service calibers are so close to each other in terms of ballistics that it's negligible between the 4. Find a weapon platform, ammo and practice until you feel comfortable and can be effective in deployment in a life or death situation.
The ONE THING we have to remember is this - Your deployment of a weapon is due to an imminent fear or threat. You use the level of force to STOP THE THREAT. There isn't a "rule" in a gunfight that says you can only shoot X number of rounds. If you've shot once, and the threat is still active...shoot again. If it requires 3 more shots, that's what it requires. As long as you can articulate why you did what you did, and it's legally justified, criminally speaking you should be ok*.
*I'm a Texas LEO in a predominately Red county. This is my personal opinion and observation after being in LE for almost 10 years between Jail and Patrol divisions. I can add a whole lot more like be prepared to have your weapon seized, hands bagged, possibly handcuffed and interviewed a few times. But anything "specific" to how a criminal investigation MAY work, will depend on your local agency/their policy and procedure (not to mention political as whether it's a Blue state/city/county vs Red etc).
