9mm is not the only caliber of pistol round. There are pistol calibers out there that have as much energy as a .223. It also doesnt say anything about ammo type. FMJ? Hollow-point? You mention large mags... it takes an unskilled, uncoordinated person maybe 2 seconds to swap a mag. Lets take the Nashville shooting as an example. The cops did an absolutely fanfuckingtastic job of getting in and taking the idiot out, but it still took them 12 minutes to arrive. Thats 720 seconds. Do you really think 30 seconds for 15 mag swaps will make a difference? If nobody is fighting back, and they have all the time in the world, the amount of ammo they can carry in a single magazine is meaningless.

Guns in war have not evolved to kill large amounts of people. Thats what we have bombs and missles for. They've evolved to protect the person wielding it. The higher the velocity, the further away you can be from your enemy. The more rounds in the magazine, the more you can spam towards the enemy in hopes of hitting them while not getting hit yourself, as you likely wont have time to take a steady shot.