Some thoughts. This isn't an exhaustive list of why things can get nasty on gun forums.
1. A lot of what people believe is obvious fact about firearm is in reality, very difficult to demonstrate in the real world. This results in people who think they know the truth but really are just stating opinions. When people disagree, it's hard to be civil when you "know" that the person arguing with you is contradicting "obvious facts"--just as it is to be civil when you know that the person arguing with you is making statements that can't be proven and is only repeating nonsense they heard elsewhere.
2. The physics behind firearms is not well understood by many, and its significance is therefore commonly either over- or under-stated and it is commonly misinterpreted. This results in people who think they know the truth but are really just stating opinions. On one side of the debate you have people who are angry because their perception is that they're being called ignorant/uneducated and on the other side you have people who are frustrated because they understand the science and yet they can't make any headway in explaining it to people who don't/won't understand. And there's a whole spectrum of people in between who get parts of it but not other parts.
3. Weapons have, historically been thought of as extensions of the warrior. The people who wielded them used to commonly ascribed magical properties to them. Although we are far more enlightened these days, the same desire to offload responsibility to the weapon still exists, especially when scientific basis for firearms performance isn't well understood, creates dogmatic/fanatic beliefs about the superiority of certain weapons/calibers/brands. We won't come straight out and say that they're magic any more, or that they were given to us by a deity, but when it comes right down to it, there's still some of the desire to "mythologize" our favorite guns. Comments we perceive as attacking those firearms/calibers/brands are often taken as heresy, in almost a literal sense. Also, in the same way that negative comments about our religious beliefs are taken as personal attacks, negative comments about our weapons tend to be taken as personal attacks. On the other side, there's frustration about those who hold strong beliefs that aren't factually based.
4. There are a lot of traditions about guns and a lot of "history" handed down from previous generations and those who we respect. When people don't agree with our opinions about firearms, we often see those comments as attacking the people who helped us form our opinions. On the other side there's frustration with those who won't/can't accept that not all the information handed down was factual.
5. Humans are extremely susceptible to various biases and, to make it worse, are often unaware of their biases. In addition, humans are extremely good at rationalizing their actions--to the point that they may believe their own rationalizations even when those rationalizations aren't really based in fact. So, the gun I bought is the best choice, not just a gun that looked really cool to me, or one like my dad owned, or one like the actor used in my favorite action movie or TV show, or the one I could afford, or one I saw in a magazine when I was first getting interested in guns, or the latest one I bought that I really, really like a lot right now, etc. It's hard to stay civil when the facts being argued aren't really the facts that mattered. Factor in #3 and #4 and things get even touchier.
6. People tend to have a hard time understanding the limitations of experience. Common fallacies based on experience are: "It's never happened to me so it can't be an issue even if other people or statistics say it's pretty common." or "It's happened to me so it must be an issue even if other people or statistics say it's actually pretty rare." Plus, personal experience is, well, personal. It's hard to be objective when someone makes comments about something that is personal.