To add to what Hasaf posted above, there are so many failures in a well-implemented self-protection strategy that must occur before defensive-firearm caliber even comes into play.
To name a few:
There must be someone seeking to do evil lurking about.
The problem with this is at any given time in an urban and suburban area there are people who are upset enough to physically attack someone (road rage would be a classic example) or who have a criminal intent who are around you.
Short of moving away from other people and living in a remote location (which then brings its own challenges) you can't really eliminate this.
You must be in the same vicinity as him.
He must be equipped to carry out his potentially-deadly plan.
Again, in an urban or suburban area there will always be these people about. It's just a matter of how much they hope to gain (money, street cred, bragging rights, attempting to dissuade you from talking to a certain girl or attempting to drive you out of a certain area [territorial dispute]) by attacking you and whether that's worth the consequences (prison, potential injury suffered in the attack or loss of face if they lose).
Honestly all it takes to attack someone is decent conditioning and a pair of fists and feet. Or a pot metal knife bought at a gas station. They don't necessarily have to gear up with a carload of guns, ammo, body armor and spare mags.
He must select you as a viable target.
Depending on the situation you could honestly just be in the wrong place at the wrong time. You could be the wrong ethnicity in a certain neighborhood, you could have answered the question of a girl who has a jealous boyfriend sitting nearby, you could be driving a nice car and be dressed decently and appear to have money/jewelry/nice watch and a wallet full of cash and credit cards. There are a multitude of reasons that some people attack other people.
It doesn't even have to be a valid reason to you. Violent offenders do not think like normal people. They attack others for very little reason.
He must get to within close enough proximity to you.
This is not like in the Stone Age where the members of two different tribes bumped into each other in the forest and both immediately assumed an aggressive stance until they were reasonably sure that the other wasn't going to start chucking rocks and spears or come at them with an adze.
It's also not like a violent person is going to be walking toward you waving a flag and yelling that they are going to attack you and attempt to rob you.
In society people let other people approach them. In order for society to work people let strangers walk by them. You interact with strangers every day. There has to be a certain amount of trust for civilization to function. Criminals are good at using this to their advantage. Most people are too busy playing candy crush on their phone to pay attention to their surroundings.
Having said that there are indications that others are up to no good.
You must fail to observe/confront/challenge him (if he "gets the drop" on you, caliber won't matter.)
Some people simply don't know what to look for to anticipate an attack. Here are some examples of some 'Pre-Assault Indicators'.
25 Pre-Assault Cues
Circling
Balled up fists, aggressive language, stance, foot placement, eye contact ('Mad dogging')
You must fail to draw/deploy your weapon (again, caliber won't matter.)
If you do draw, he must not be deterred.
Lets skip to the last video I posted. The victim exited the store and was immediately 'in it' and was confronted by the enraged man looking for a fight. I'm going to guess that he hadn't heard the enraged man ranting and threatening others before leaving the store. Otherwise I imagine that he would have either stayed in the store or would have gone out the other way.
Once he left the store though and was immediately confronted by the man the second, third and fourth things that he could have done would have been to ..
.
• 1) Not engage with the guy at all and either go back into the store while also 2) readying himself for an attack and getting better foot placement, getting ready to go hands on/use of grappling and boxing skills, 3) getting some distance from the guy to gain some reaction time, 4) ready the pepper spray (if he has any) or perhaps get ready to draw (depending on his grappling and boxing skills, age, health status etc)
Or
• Attempt to put himself in a better position to defend himself from and go around him while stating that '
I don't want any problems' and ready yourself for an attack mentally while going for your vehicle at a steady pace and non-verbally suggesting that he might be biting off more than he can chew.
Just saying ... attacks aren't always preceded by flags, horns and big flashing lights. There are usually indications (looks to accomplices and glancing around for exits, police and people who might aid you, movement towards you, a change in stance or conversation, physical indications of anger or getting ready to attack), but you have to be paying attention to that stuff.
If you carry a gun there's also at least one gun on the scene. If you fail in getting some distance from an 'unarmed' attacker ('unarmed' is a word that the media throws around a lot) or perhaps getting something between you (vehicle, table, display, gas pumps etc) and he gains the upper hand in an altercation
YOUR gun can quickly become
THE gun and it could be used against you.
If you fire (miss or not), he must continue to "soldier on" (not flee, surrender, or be neutralized.)
Usually, yes. Most often once there's gunshots everyone scatters, that also usually includes the attacker(s).
However much of this stuff takes place at contact/bad breath & body odor distance and in that case they might believe that their best chance of not getting shot (if you missed) or shot again is to disarm you and/or to continue stabbing, shooting or beating your head flat.
That's generally the reason why retention training is important to anyone who has decided to carry a pistol. Training with South Narc comes to mind, but some escrima and jujitsu classes have it incorporated into their program.
At this point, caliber may begin to play a factor.
To some degree, yes. No one wants to be shot with anything and until rounds are actually fired caliber and load don't play a part in that.
I have personally seen some offenders back off merely because the pistol displayed or shoved in their face was large, but maybe mentioning that was a saving face gesture on their part on why they backed off.
I get that not everyone is willing to put up with dressing around the pistol or the slight discomfort of IWB or appendix carry. I carried around full size pistols (BHP's, Beretta 92FS and CZ-75 Compact) around until roughly 3-4 years ago and even I got sick of it.
There's a whole crop of 9's and .40's and even some .45's that are roughly the same size as some of these .380's. However if the largest pistol that some can conceal well and that they'll actually carry regularly is a .380 then so be it. At least they've taken responsibility for their own safety and they're trying to do something about it.