Another thing to think about , the first round if it hits is more importan than 20 that miss and if the misses per round have more Foot lb of energy it is waisted.
Also if you are going to carry ,the size of the gun can make a differance, I carry a Kel Tec P-11 9mm ,I feel that I can hit with it if needed .
Many guys don't shoot enough or they are shooting in the wrong enviremont , to hit any thing in a panic .
1 9mm will do the job if it hits the target , 10 40 s&w rnds missing the target are a waste.
Shoot what you can hit with