Its not that simple. Most people see suicide as a fix for their problems, firearms are used a lot because its a very simple and quick fix. If a person has a outlook on life that things aren't going to get better, they are severely depressed, have health issues, etc they might choose to kill themselves. Most of the people I know that killed themselves weren't in debt, they didn't have that bad of lives, and overall seemed to be doing ok. They weren't happy, and had mental related issues (severe depression).
Depression and other mental illnesses can alter a persons perception of reality. They can also make a person absolutely miserable to the point they literally can't function in day to day life. Its hard to say that they are insane for doing what they did, because they chose something that they felt would end the problems.
Imagine being in a linear world where choices exist to fix your problems (such as seeking help, asking others for help, etc) but you are almost completely blind to them. You start making things worse by constantly making bad decisions, and every day that goes by you feel worse and worse. One day you awake to realize that you are so far in a hole that its unlikely your ever going to get out. You realize that you can't ask others for help because A) others might criticize you and not care and B) there are serious repercussions for admitting you want to kill yourself (firearm ownership rights, your employment, etc). You can't help yourself, your afraid to ask for help, and your want to end your suffering. If you feel that bad about yourself its easy to see why you could think everyone would be better off without you. Thats what a friend of mine basically said in his death letter.
The hardest part of understanding suicidal people is that its not what they are physically dealing with, such as big debts, loss of family members, terminal illnesses, etc but how they FEEL that makes them do what they do. Your average person could cope with loosing a job, being late on a rent payment, and even having a car break down, maybe even all at the same time. A depressed person (or a person with other mental illnesses) could feel like they lost all of the people they loved, that they are worthless to the extreme, etc over something as simple as being late to work. Feelings of very strong terrible emotions to simple things is common.
When it comes to shooting buddies and suicide, it is a real sticky situation because you don't want to alienate your buddy, but you don't want to see them do something stupid either. You have to pay attention to important signs such as commenting about how things would be better if they were gone, them giving away prized objects, etc. You have to keep and open mind and realize that its not what they are dealing with such as bills/spouse problems/etc but how they are coping with their problems. Don't be afraid to sit down and have a serious talk about suicide, and how they need to call you if they are thinking about it.