My friends and I did this last year in a big, empty lot. But it was just 4 of us. And our weapons were just gun noises with our mouths (plus engine whines during tricky maneuvers, of course). And you pretended to get hit when you felt like it.
i used to play a game called “battle bike”, does anyone play that? no guns – the only real rules are if you go out of bounds or put your foot down, you lose. after that, it’s up to the participants how nasty things get.