While an open-world game definitely needs to have an engaging setting and plenty of activities to keep the player occupied, it should also have a satisfying combat system that manages to feel both fun and challenging at the same time. In the case of first and third-person shooters, this means making the guns themselves as believable as possible, but considering there are so many factors that go into this, it can make it a pretty difficult task for developers to make the moment-to-moment gunplay in their games as exciting as they would like.