I think it's a hard sell if the dog is shot on the owner's property. You may assume it's going to attack but you know what assuming does. If you have time to pull a gun you have time to begin backing up to see how it plays out.

 

That said, if someone has a large dog they should not allow it in the front yard off of a leash, invisible fence or not. Way too many things can happen.

A. I have to assume it was a dog that by it's size alone would make one afraid.

B. If I was on public property, i.e. sidewalk, I'd move far enough away that I wasn't as near the property line.

C. If A and B are followed by the dog proceeding towards me in a menacing fashion, I'd have split his head like a canoe.

D. If A,B,C are all over, I'd live with the consequences. I'd wait for Doomer to read the paper the next day and tell me I was a coward because I was carrying a weapon as I was in jail or in the leisure of my own home unscathed by a dog's bite.