When considering what caliber to carry, you may want to try various brands of ammo in your caliber.
Some brands produce more muzzle flash than others, something to consider in low light situations.
All you need at night is a good flash to make you blind when trying to defend your life.
As for Caliber... I like the 45acp Remington Golden Sabre.
Statistics show most gun fights end after 6 shots, and are at a distance of less than 10 feet.
Your going to be in a high stress situation, adrenalin surging through your veins.
Not like target shooting at the range, so you wont have time to slow down and take carefull aim.
Most likely you will be shooting instintivly, and you will most likely miss with most of your shots.
Cops tend to miss as much as 80% of their shots in a defence situation.
So choose a caliber that will do the most damage when it hits during the 20%.
Also, choose something that will allow you to bring sites back into alignment faster after each shot.
Recoil will slow your reaction time and keep you from getting off as many rounds as you need in the time of the incident.
Practice makes perfect.
Practice drawing from your carry rig and shooting a shot instintively, so that it becomes a natural action.
You wont have time to think, only react.
Get to where you can feel (use the force luke) where the bullet is going, in case site picture is not possible.
Practice shooting from the hip, you might only be able to just clear leather before needing to fire.