For a paint-ball gun: small overripe tomatoes or grapes; mini-marshmallows might work if shot out high-powered, but would be low on the splat factor unless dipped in something. We had an ... interesting idea - take a hot dog and chop off the ends and glue the ends together (pick your means of doing so) and you have created a "bullet". The remainder can be used for the BBQ.
If you use a water-gun - chill it and fill with whipped cream.

Water balloons filled with Cool Aid - lovely coloring... NB: Green Cool Aid in blond hair does not necessarily wash out easily ::snickers::