It's a nerf gun dart tip that explodes! 
Just insert one of those little fire cracker things kids throw on the ground (the ones you see in the picture), and it will go BANG on impact!
I made vent holes at the back so that you can actually hear the bang.
so far after at least 10 bangs on multiple tips, they are still whole, except for some impact damage from hitting a concrete wall multiple times.
Just glue them into the “body” of old nerf darts.
i printed them tip up, in pla, with some supports, in just a few minutes.