The teaching of the Second Vatican Council's constitution Gaudium et Spes is that those who enter military service, and carry out their duties in a proper manner, "contribute to the maintenance of peace" (cf n.79). This implies a legitimacy to the reasonable production of armaments to enable them to do that task. It does not, however, legitimise a massive commercial production and export of armaments. In the context of the former confrontation between the nations of the West and the Soviet bloc, the same constitution taught (n.81):(UK Defence Secretary) Mr Fox will say at an arms exhibition in London he is "proud" that the UK is the world's second biggest defence exporter and that helping the firms prosper is in the national interest. [Source: BBC News]
... the arms race is one of the greatest curses on the human race and the harm it inflicts on the poor is more than can be endured. And there is every reason to fear that if it continues it will bring forth those lethal disasters which are already in preparation.More recently, and writing in the contemporary context, the Pontifical Council for Justice and Peace has observed that (with my emphases added in bold):
Weapons cannot be considered as any other good exchanged on the global, regional or national market. Their possession, production and trade have deep ethical and social implications and they must be regulated by paying due attention to specific principles of the moral and legal order. Among the principles there is the principle of sufficiency, which allows States to possess only the means necessary to guarantee the legitimate protection of their people. [Statement on behalf of the Holy See October 2006]And referring to some of the types of weapons systems likely to be on show at the DSEI, and likely to be sold to more sizeable overseas markets:
The Church's social teaching proposes the goal of “general, balanced and controlled disarmament”. The enormous increase in arms represents a grave threat to stability and peace. The principle of sufficiency, by virtue of which each State may possess only the means necessary for its legitimate defence, must be applied both by States that buy arms and by those that produce and furnish them. Any excessive stockpiling or indiscriminate trading in arms cannot be morally justified. Such phenomena must also be evaluated in light of international norms regarding the non-proliferation, production, trade and use of different types of arms. Arms can never be treated like other goods exchanged on international or domestic markets....[Compendium of the Social Teaching of the Church n.508]
Appropriate measures are needed to control the production, sale, importation and exportation of small arms and light weapons, armaments that facilitate many outbreaks of violence to occur. The sale and trafficking of such weapons constitute a serious threat to peace: these arms kill and are used for the most part in internal and regional conflicts; their ready availability increases both the risk of new conflicts and the intensity of those already underway. The position of States that apply severe controls on the international transfer of heavy arms while they never, or only very rarely, restrict the sale and trafficking of small arms and light weapons is an unacceptable contradiction. It is indispensable and urgent that Governments adopt appropriate measures to control the production, stockpiling, sale and trafficking of such arms  in order to stop their growing proliferation, in large part among groups of combatants that are not part of the military forces of a State.[Compendium of the Social Teaching of the Church n. 511]When the DSEI exhibition took place two years ago, I was able to take part in the protest vigil that took place the evening before the exhibition opened (see here, and scroll down to the report dated 8th September 2009); the same page reports the protest vigil that took place on 12th September 2011, with Bishop Thomas McMahon of Brentwood Diocese as one of the participants. The Defence Secretary's reported "pride" in the UK's status as the second biggest defence exporter (or, to use a more honest term, as the second biggest arms trader ) is, I believe, at best misplaced and at worst abhorrent.