Over 135 firearms and 7 000 rounds of ammunition seized are a few of the final results of Operation Calibre, a series of international actions in 10 EU Member States and 8 third parties, led and coordinated by the UK as the Activity’s Action leader and with the full support of Europol and Frontex. Initiated in September 2017, Operation Calibre culminated in a two-day joint action on 17-18 November coordinated from Europol’s Operational Centre in The Hague during which physical checks were carried out, mainly in the Western Balkan region. Europol’s Operational Centre saw participating authorities exchanging information on people and vehicles being checked at border crossing points, seizures made and ongoing investigations. The UK, Sweden, Croatia, Slovenia, Bulgaria, Greece, Romania, Spain, Hungary, as well as Switzerland, Albania, Bosnia, Former Yugoslav Republic of Macedonia, Serbia, Montenegro, Kosovo*, US ATF, Europol and Frontex all took part in this series of actions. During this two-day action, 18 individuals were arrested in the Western Balkan states in relation to weapons and explosives offences and 153 individuals were prevented from entering the EU illegally. In addition, law enforcement identified a further 37 instances of overstay. 4 Europol mobile offices were deployed to the Western Balkan region and the UK, allowing for real-time information exchange and cross-match analysis of the data collected. Officers at different border crossing points in the Western Balkans were facilitated on the spot by Frontex. Operation Calibre is part of Operation Dragon, the fourth EU-wide set of Joint Action Days taking place within the EU Policy Cycle for organised and serious international crime and was planned under the EMPACT framework for firearms.