My rules for combat (guns, hands, whatever, these apply):

1. Have a plan and train that plan
2. Make the plan simple
3. Know that your worst in training will be the best you manage under stress (in other words when you have a bad day in practice - that is the standard you can rely on under pressure)
4. Mental skills beat hardware
5. Luck is always a factor - by entering into combat you have invited the gods of chance to play, and there is always something that can go wrong (slipping on a wet floor, unlucky ricochet, misfire whatever). Training and hardware can minimise it, but they don't eliminate it, so don't enter in to it lightly