You aren't judged by information available after the fact. You have no way of knowing its a prank until after the fact, and is thus irrelevant for determining if the shoot is reasonable or not. Legally, I mean, in the court of public opinion you'll be scrutinized with 20/20 hindsight, but not by the police and by the court system.
I'm think'n he juuuuuust 'bout open'd a can o' worms with dude at gas pump.......
That's what I thought too.
I don't want to live in a country where you can't shoot clowns.
EDIT: .....and mimes.