The truth is, there is little the Church has ever said about this to expressly answer this question. I’d like to believe one could have a legitimate reason to own a firearm to protect oneself. Like the issue of “The Death Penalty”, each issue would be weighed separately to be able to make a moral judgement, hence the importance of consulting a spiritual director or any wise priest/theologian.
The Church through the Delegation to the International Conference on Illicit Trade in Small Arms and Light Weapons in all Its Aspects say this:
Unfortunately, however, it is impossible to ban all kinds of small arms and light weapons. In a world marked by evil . . . the right of legitimate defense by means of arms exists. This right can become a serious duty for those who are responsible for the lives of others, for the common good of the family or of the civil community. This right alone can justify the possession or transfer of arms. – July 11, 2001