I don't think his defence as stated will stand. A gun that isn't there is not a threat.
It doesn't matter whether or not a gun was in the car, as long as Dunn thought there was. To use deadly force, Dunn would have to prove that he REASONABLY believed he was in danger of death or serious bodily injury at the time he fired the shots. If he reasonably thought that one of the teens was armed and that he would be shot if he did not act when he did, then under the law he acted in self defense.
In self defense, what the defendant reasonably believed is even more important than the facts. For example, suppose that John and his friends planned on playing a prank on Tim. John was to approach Tim with a gun and threaten to kill him for messing with his girl. John acts as though he was angry to carry off the prank. However, unknown to John, Tim was lawfully armed and he pulled out his gun and fired into John's chest killing him. In fact, John never intended to shot Tim but under the law John's intent doesn't matter in the least. In fact, Tim was never in any danger but since he reasonably thought he was he was allowed to use deadly force in self defense. Even if John had used a toy gun Tim still had the right to shoot in self defense if he reasonably thought the gun to be real.
In the present case, if Dunn can prove someone in the car had threatened to kill him and he saw what he reasonable believed to be a gun in someone's hand, he might be able to claim self defense. However, under the law it is not enough for Dunn to merely
say he thought he saw a gun. Rather, Dunn must
prove that he reasonably thought he saw a gun. It is the function of the jury to determine whether or not to believe Dunn. If there was nothing in the car which reasonably resembled a gun, Dunn is done (pun intended).
The most troubling fact is that Dunn pulled the trigger 8 times. That makes it appear he wanted to kill as many people as he could, and that is inconsistent with a claim of self-defense. I am curious what the shot patterns will reveal.
I need a lot more information before I can predict what will happen. However, based upon the limited knowledge I have, I doubt that deadly force would be justified if Dunn is unable to prove he truly thought he saw a gun. Mere threats do not give rise to the use of deadly force. It is the threat coupled with the capacity to carry out that threat that justifies deadly force. I can't see how a car full of teenagers, even angry ones, justifies deadly force. Now, if after threatening to kill Dunn the teens got out of the car and approached him that is another story. However, as long as they are sitting in the car, they are not a present danger to anyone.
I seriously doubt that Dunn is going to get away with this. I strongly expect a plea deal of some kind. I also anticipate a wrongful death civil action.
Just my own humble opinion.