The primary quality of the defensive handgun is reliability. As I wrote elsewhere It must work ..if it doesnt everything else, accuracy, ergonomics etc are irrelevant. The pistol must function reliably.
How reliable should the pistol be? 100% reliability is not attainable in any mechanical instrument made by man, and 99% is unacceptable. I wouldnt own a weapon that failed with one shot in every hundred. So we try for as near to 100% as possible. So lets devote this topic to reliability.
The first thing is to choose a weapon with a proven track record, one that has been tried and tested and has gained a good record for reliability. Sadly, there are some weapons with inbuilt design flaws, which will never work properly. These we discussed here.
Of course, an individual example of an otherwise reliable design can be undependable, so its essential to road test any model before carrying for self protection. You should put about 200 rounds of your chosen ammunition through the weapon before relying on it.
This brings us to the next concern:
There is a tendency for some shooters to carry a magic super-exotic type of ammo, guaranteed to stop a charging buffalo with one shot, without ensuring that it feeds and cycles consistently in their particular pistol. One reason is that exotic ammo tends to be expensive, and putting a couple of hundred through the pistol is a significant cost….. but consider the cost of failure.
Ammunition must suit the weapon. Bullet profile, recoil impulse and gas pressure produce an individual set of characteristics which must match your weapon. Also, performance may be flawless when you are locked in, two handed, weight committed, but in a worst case scenario you might be firing one handed, from an off balance position, with a sweaty or bloody hand. Under these circumstances will the weapon still cycle that ammo?
These are the source of the majority of reliability problems, and Ive discussed this in depth here.