11B20 There is a lot of very interesting stuff on that gun battle on the web, you can find many hours of interesting reads,
But in a nutshell, the 2 BG's got shot early on. One in the head, where it bruised his brain and for the most part took him out of the action. The 2nd BG took one through the arm and into the chest, but the bullet stopped 1" from his heart. The wound was fatal but he was able to fight on and kill 2 or 3 FBI good guys in the next 4 min. of the battle. A little more pop and he would have had a hole in his heart.
The shots were kill shots, but the ammunition didn't have the penetration needed. That's when the FBI went to the 10mm, but the recoil was to much for some, so they wanted to go to the lighter load (FBI 10mm). S&W wanted to get rid of the empty space in that load and went to Winchester to make a new cartridge, and that's how the .40 S&W was born.
I guess the upshot is you need both.