A 100 year old AA gun has been restored by English Heritage and is now operational at Dover Castle. If you are planning to visit the castle the gun is fired twice a day on weekends.
This is a 3" 20 CWT purpose built AA gun of the type that defended Dover during WW1.
Here's a video of a firing:
https://www.youtube.com/watch?v=z-3h2AOurJ0
Bruce, #3 on the gun crew.