OP’s question has already been answered, but I wanted to address people’s practical concerns. Realize that I live “out-of-town” in a State that doesn’t have NY/NJ/MD’s restrictions and practical inability to obtain conceal carry permits.
1. I absolutely agree that not every Chaim and Yankel should just “go out and buy a gun.” First of all, not everyone is the right person, i.e. if someone struggles with ka’as, isn’t good at ascertaining situations, etc. This is true of anything. HOWEVER…
1a. In most of these horrific situations, the rasha is a coward who will run at the first sign of any resistance. So even if the mispallel misses, or, chas v’shalom, hits someone else, it will generally lead to the rasha running away and will save lives anyway. Ma kol shekein if there’s several people.
1b. The bigger concern with lack of training is just someone not carrying properly and leading to a negligent discharge, i.e. doesn’t have the right kind of holster, isn’t familiar with the different aspects of the pistol, etc.
1c. The NRA has a good online course that gives people- especially people with 0 prior knowledge- a good grasp on the very, very basics that should help prevent most “Chaim and Yankel” problems. (I don’t care about their politics, I just know that this course is a good intro course.)
2. Becoming decently proficient does not take a lot of time, but costs can add up, especially in-town where ammo is much more expensive, and you can’t order in bulk because no one will ship there due to the aforementioned laws and regulations. So before someone decides to do this, they need to be able to budget between $30-$50/mo for range time and ammo.
3. People from in-town often seem to have the mentality that Reb Eliezer has, that regular people are not trained to use firearms. However, as Chaim Eliezer alluded to, the truth is that most responsible gun owners tend to do much, much, MUCH more training than the average police officer. Most police departments only require officers to qualify once a year, whereas, as I mentioned above, you should go to the range at least once a month, if not twice monthly. Will it make you the bulls-eye shooter that Jack Wilson was after owning a shooting range and being a firearms instructor for decades? Obviously not, but like I said above, that’s not what you need in most circumstances, whether in Shul or at home.
4. Akuperma brings up the Mishna in Shabbos. Several people in my Shul carry, and besides for a few who like to talk about it, I’m pretty sure no one besides the Rav knows the name of everyone who carries. Conceal carry works- again, when you take the couple of hours to actually learn how to do it, and not just assume you know. It’s really not that complicated, but that initial training/book reading MUST be done.
5. The other big thing that people don’t think about is the law. If you don’t know the laws of self-defense, you can easily find yourself the suspect instead of the hero. You must learn the laws of self-defense in the State you live, possibly by reading the book “The Laws of Self Defense” by Andrew Branca.
5a. Because of the legal concerns, especially in States that don’t like guns to begin with, I would highly recommend having carry insurance. This insurance, which can buy from the USCCA, U.S. Law Shield, and other companies, will cover your legal fees should you be charged and prosecuted after a shooting.
I guess what I’m saying is that people who aren’t familiar with guns drastically overestimate how much training and knowledge is required, however the basic stuff really should be done, even if the law doesn’t require it. (And no, even NY/NJ’s required courses don’t qualify.) Guns are not magic kaballah talismans that will ward all harm away from you. They’re powerful tools that, when used properly (or even owned properly) can help save your life and the life of others.