1942 Bofors 40mm Anti-Aircraft Gun USA
This Anti-aircraft gun was in use for quite a while, after WW2, seeing service in the Korean and Vietnam war. However, the ascendence of jets to a lot of roles on the modern battlefield has rendered these guns as relics of the past. It can't kill what it can't aim fast enough at. Most modern airdefence systems are now based around Surface to Air Missiles, and those that are based on guns have a sophisticated tracking system built in to increase the likelihood of a kill on slow-moving targets such as helicopters and ground attack aircraft like the A-10 Warthog.