I’m not sure why there is, or should be, any question about firearm usage in self defense. The old adage, “God created man, Sam Colt made them equal.” has never been seriously disputed. All you need to do is consider a “fight” between the average male (180 lbs) vs the average female (110 lbs). Disparity of force makes this an unfair fight; some equalizer is clearly necessary.
IF there is any dispute, it is between “train” and “not train”. Unfortunately the split seems to fall between those who train advocating training, and those who don’t advocating not training. Self defense is a somewhat nuanced area, and being familiar with the numerous loopholes and pro’s and con’s of using a weapon would seem to unbalance the equation towards training. Yet no where near half of those who have a firearm for self defense are even aware of the multitude of things that have to cross your mind before you choose to use a firearm in defense of yourself or others. Sometimes the situations are pretty clear-cut; often not. Knowledge can make the difference.
First and foremost, you need a certain level of skill in using a firearm. Handguns are, by far, the most difficult to learn to handle properly - one reason there are numerous “gunfights” on record with a multitude of shots but no hits. Handguns have a multitude of degrees of freedom in their deployment that make technique important. They are also the least powerful of the firearm groups, an obvious fact else militaries would be solely armed with pistols. The fact that rifles are the dominant firearm in any military says all you need to know about lethality. Handguns are common because they are portable and easy to access. If you KNOW you are going to a gunfight, OTOH, bring a long gun - any long gun. The much maligned EBR (Evil Black Rifle) fires a moderate range cartridge known for its low recoil and generally small size. Yet Rittenhouse dispatched three large assailants with 4 shots - two dead, one never going to use the arm he held a pistol in, well again.
Second, you have to have a willingness to use the weapon against another human. This is not nearly as obvious as one might think. A LTC Grossman has an outstanding book called On Killing that clearly shows that the majority of people have a hesitatin on using a weapon - of any kind - in lethal defense of their lives. “A species doesn’t (usually) kill itself”, to quote him. So IF you decide you are going to have a firearm for self defense, you need to train in a certain way to help insure that if/when the moment comes, you are prepared to not be another victim and instead defend yourself vigorously - and effectively. Training - there’s that word again.
The rest of the considerations are minor in nature - type of weapon, penetration of ammunition, characteristics of firing the weapon, loss (momentary) of orientation in a gunfight (usually due to focus and noise/temporary deafness).
Lastly one must always remember that the human body can absorb an incredible amount of damage and continue to, especially, fight. A shot THROUGH the heart - can leave the assilant with 6-10 seconds of conscious fight left. THAT, folks, is an eternity in a gunfight. Consider that back when I competed in IPSC I could shoot something called an El Presidente (double tap 3 separate targets, reload a mag, and repeat double tap three more targets) in 4.1 seconds. So technically I could be shot through the heart and still kill 6 people - before I dropped dead. I am dead; I just don’t know it.
Post script: Know the law. You carry a gun so you’re hard to kill; know the law so you’re hard to convict (legend od Andrew Branca, self defense expert lawyer).