An increasingly common topic in Catholic publications is whether and how Catholic Social Teaching applies to the problem of gun violence. One compelling theory is that the ownership and use of firearms should be governed by the principles of the Just War. It is suggested that, by analogy to the Just War principles which govern whether and how countries go to war, a moral framework may be created to determine when it is morally appropriate for individuals to use firearms, and to determine how firearms should be used once that decision is made. It is perhaps useful, then, to revisit the Just War doctrine and its criteria.
The Catechism of the Catholic Church (“CCC”) teaches the doctrine of Just War, principles which apply whenever a nation is considering use of force to defend itself against an aggressor.  The doctrine was first enunciated by St. Augustine of Hippo in the 4th Century AD, and expanded by doctors of the Church, particularly St. Thomas Aquinas, and then formally embraced by the Magisterium.
The foundation of the doctrine starts with the idea of righteous anger. Anger in and of itself is not evil, but only ordered or righteous anger justifies the use of force. Anger is a desire for revenge, the passion or emotion by which we react to evil, whether real or imagined, and seek justice and vindication of rights. Ordered, or righteous anger is directed toward a legitimate object, with only the appropriate level of vehemence. Disordered anger, on the other hand, seeks evil only for the sake of evil.
An example of righteous anger is that of Jesus in the Temple. His anger in hurling tables and using a whip was directed toward legitimate ends, driving the moneychangers out of the Temple and prevention of their return. His anger had a specific end and was not expressed for the sake of violence.
The goal of righteous anger should be the restoration of peace and justice, and actions should not knowingly reduce peace or less justice.  Violence should only be the last resort. All citizens and all governments must work for the avoidance of war, however, leaders who are responsible for the welfare of others have an affirmative responsibility to use force to obtain justice. Leaders must exercise this responsibility once all efforts at peace have failed.
In addition to the above conditions, the following criteria apply to the use of force:
1) the damage inflicted by the aggressor on the nation or community must be lasting, grave, and certain
2) all other means of putting an end to it must have been shown to be impractical or ineffective;
3) there must be serious prospects of success
4) the use of arms must not produce evils and disorder more grave than the evil to be eliminated.
The responsibility for determining these conditions are met belongs to “the prudential judgment of those who have responsibility for the common good.”The doctrine also forbids, in the context of war, attacks or mistreatment of non-combatants, genocide, and the indiscriminate destruction of vast geographical areas or cities.
Underlying all these directives is an overall responsibility to work to reduce the underlying causes of war, such as economic inequality, distrust, and pride  As developed through scholarly tradition, the doctrine can summarize as follows: war must be a last resort waged through legitimate authority; there must be the right intention and probability of success; the violence must be proportional to the casualties suffered; and the use of force must distinguish between
 “The Church Has a Stance on War, But Needs a Stance on Guns,” Marvin Lim, National Catholic Reporter, December 1, 2016; “Catholic Response to Gun Violence is Contradictory and Confused,” Ken Briggs, National Catholic Reporter Online, April 12, 2018
 CCC ¶2302-2303
 CCC¶ 2307-2317
 CCC¶ 2309
 CCC¶ 2317