This is a fun weapon we used to make in grammar school. It cost very little (less than $1) and is very versatile. Depending on the ammo it can merely sting or potentially draw blood (don't aim for the head, any ammo could put out an eye).
Step 1: Materials
1- 25¢ fruit flavored drink (or any wide mouth plastic bottle -around 2in)
2- sturdy mid to large party balloon
3- rubber bands
5- ammo (skittles or m&ms for fun, bb's or ball bearings for real damage)
Step 2: Prep Work
Grab the neck of the bottle as if you were making an OK sign with your hand.
Cut off the base of the bottle about an inch and a half from where you are holding it.
Discard base or use it as storage for your ammo.
Step 3: Assembly
1- Stretch balloon over opening an keep sliding it over the opening until you are as far as the widest part of the balloon.
2- Tightly wrap rubber bands around neck of bottle to secure the balloon.
3- Congratulate yourself on a job well done.
Step 4: How to Use
1- Grasp neck in familiar OK grip.
2- Place ammo into balloon. Don't use rocks, they'll make the balloon wear out faster.
3- Grip ammon through the balloon and pull back.
4- Point opening at target and release.
At short range a Skittle or M&M fired at full draw may leave a welt if it hits bare skin. A bunch of birdseed will spread like a shotgun. Anything with a rough edge can draw blood (a ball bearing might as well, but I haven't tested it). Once again, don't aim near someone's eyes (it's pretty accurate but why risk a permanent injury).
If the balloon wears out, just replace it. It makes a loud snapping noise when fired so it's not a stealth weapon. Stronger balloons make for a more powerful impact.