I'm not a Psychologist, though I have taught the subject. Some basic themes are evident.
Projection:
A good deal of 'projection' is inherent to the anti-gun belief system. People tend to project thier values, beliefs and worldviews onto others, and presume that those others would deal with a situation the same way they fear that they would (i.e. "if I owned a gun, I would kill people when I got mad... therefore anyone who owns guns might do the same thing, therefore they should not have guns").
Schemas, Accomodation & Assimilation:
When people's belief systems are threatened with new information, they tend to defend their existing schemas (knowledge structures), because accommodating a schema to new information is very difficult to do. This is why people will defend their (religious, political, etc.) beliefs assiduously even after a fact has proven the belief to be wrong. Most people will easily adapt their way of thinking on issues that they don't already have a particular belief about, but will almost never adapt their way of thinking about something they believe strongly, no matter how much evidence is provided to discredit it.
Rather than accommodating their existing schemas to new information, people tend to seek out information to assimilate into (but not threaten) their existing schemas. They find ways to make the new information fit their schemas (belief systems) rather than the other way around.
These are common defense mechanisms, and emotional topics exacerbate their effects.
Confirmation Bias:
People also tend to interpret information in a way that confims previously existing beliefs, a phenomenon known as confirmation bias. Two people can see the same set of facts and come to wildly divergent conclusions due to confirmation bias. This is why it often feels that we live in entirely seperate intellectual (and sometimes actual) worlds from people who do not agree with us.
