"People tend to hold overly favorable views of their abilities in many social and intellectual domains. The authors suggest that this overestimation occurs, in part, because people who are unskilled in these domains suffer a dual burden: Not only do these people reach erroneous conclusions and make unfortunate choices, but their incompetence robs them of the metacognitive ability to realize it."
The lower the actual skill in a particular area the more likely an incompetent person will overestimate his personal ability and be incapable of accurately assessing another's skill. This study almost perfectly models the problems found in marksmanship.
The solution is to bring more gun owners out to organized events. If you haven't tested your skill in open competition against others you simply do not know where on the skill continuum your abilities lie. Sadly, as this scholarly study points out, without this critical feedback you probably can't accurately judge for yourself.