How do you reload sentry guns in zombie assault 2

When th ammo in the sentry gun reaches 0, then you can reload it. To reload, walk on top of the sentry gun and press the F key. I'm not sure if you can reload it when there is still ammo in it though, but you can try. When you reach the rank Colonel, once you set up a sentry gun, you will have double ammo for the sentry gun, and you can reload it faster.