If it looks like the prosecutor has all of the evidence to prove up his case, then the shooter could still acknowledge the shooting and avil himself of a self-defense claim, but that claim will be a weakened one.
I'd say that this would be an understatement. Your claim will be seriously weakened by the fact that you did not report it. Not only that; but the evidence supporting your version of events may go unnoticed or uncollected without you there to help with the investigation. If you are trying to hide
your participation, it may even require evidence useful to your self-defense case be destroyed.
So in addition to the really, huge problem of immediately looking like a criminal in the eyes of pretty much every reasonable person out there - you may also lose a lot of evidence that would actually help establish your self-defense claim by not reporting the attack.