How do I make my AI attack in the FPS tutorial?

I've been working with the FPS tutorial on Unity, and my game is looking alright. I can move, I can shoot, blah blah blah. I even made some AI (the robots). They can move around within their waypoints and chase me; and when I shoot them they collapse to the ground. However, I cannot make them attack me with their weapons. So far I've tried to tactic of:

-Creating an empty object called robot gun -giving that object a machine-gun script (same one that works for the player) -dragging that object onto the robot ai

Still, none of this works. I'm probably (or obviously, since it isn't working) doing something wrong, so please give me a helping hand.