Jumping into the tank in the tanks showcase produces a hint stating the player should point at a target and press R to command fire. This doesnt work and therefore the game appears to be broken.

I would recommend it is rewritten along the following lines:

Press F to cycle between machine guns and main gun.

To command your gunner to target an enemy look at the enemy and press T.

Wth machine gun selected he will engage immediately. With main gun selected order him to fire the main gun press CTRL + Left mouse button.