I've heard lots of theories, I'll list them here for people who are interested.
Gov John Connolly was armed with a pistol, driving the car, and when he heard Oswald shoot, he turned and fired, blasting JFK with his pistol at short range.
There was a shooter in the storm drain with a rifle,
There was a shooter(s) on the grassy knoll.
There was a shooter under the overpass.
There were multiple shooters who triangulated JFK from multiple angles to make sure the job got done.
I've heard lots of theories, I'll list them here for people who are interested.