Don't get me wrong, I would most likely shoot before the dog gets it's teeth into my ass, but, I'm not sure the police\DA is going to be very understanding.
I doubt it.....if you are not on the owners property at the time of the incident then the owner is liable for any damages and is also required to keep the animal contained(this applies to counties with larger populations, I forget the number though). i.e. if you shoot a dog on the sidewalk or in the street and there is evidence of that, you should be cleared pretty easy(especially in the OkC area). The only way you might have a problem is if you shot it on the owners property....you'd have a hard time justifying your trespass.