Rear gunners work in a different way than your main weapons in World of Warplanes. You should know a few things about them.
Most importantly, they attack automatically and you cannot control them in any way. They have their own range at which they will engage enemies and they are limited by angles at which they can fire. They also have to cease fire in order not to hit their own planes’ tail.
You can make them more effective by training their perks and skills.
The rear gunners also use their own ammunition which means you cannot buy premium ammo for them. It also means, that resupplying them costs you nothing! 🙂