These "warheads" are intended to be filled with BIG pop-its! They wouldn't fit into the much simpler warhead (http://www.thingiverse.com/thing:1877583).
So I created this one, which uses a "fuse" to fire the charge. It will work on people, too, but I strongly recommend to fire it at walls and furniture only! Use at your on risk!
I printed mine without supports, but this depends on your printer. You will need rafts, though.
Print, polish, put together and glue into a foam dart.