RAF gunner training with clay pigeons - GIF

[ductape]

Ball turret guns were generally 12.7mm/.50 cal. This one looks quite a bit larger than that. The video is probably training for a 40mm gun used against the German V-1 flying bomb. The V-1 would fly for a while then drop, similar to a thrown clay pigeon.

If interested in history, watch all the British Pathe videos you can find. They are great.