Nigeria faces a high rate of illicit transnational arms smuggling, particularly in borderline areas, which poses a threat to its internal security. The proliferation of light and medium arms has led to the recognition of vigilante involvement in internal security, but the excesses are not well-regulated. This paper examines the proliferation of light and medium arms for vigilante, purposeful engagement in internal security, the acquisition process, and government control over the proliferation. The study finds that weak legislation, dating back to 1959, has exacerbated the proliferation of arms and led to endangering attitudes. The paper recommends the government revamp and develop modern firearm laws to mitigate the effects of the proliferation of light and medium arms on internal security.