You can use ping pong balls or marshmallows as amo for these cool shooters.

What you will need:
2 toilet-paper tubes (or 1 paper-towel tube, cut in half)
Single-hole punch (optional)
Stubby pencil
Pen or marker
2 thin rubber bands
Marshmallow (or small ball
How to make the plunger (i.e., the inner tube)
Cut a toilet-paper tube in half, lengthwise.
Squeeze it so it’s about half its original diameter and tape it.
Now punch two holes.Punch the holes half an inch from the end, opposite each other.
Gently push the pencil through the two holes, twisting as you push.
Tip: If the pencil holes tear… Punch two new ones. Punch about half an inch into the tube, away from the old holes. You want a good amount of cardboard supporting the pencil.

Cut slits Take the second toilet-paper tube. Draw two short lines straight down from the rim, about as far apart as the width of your index finger. Make two slits by cutting each line.Do this again at the same end of the tube, opposite your first set of slits.
Attach the rubber bands.Push a rubber band into one set of slits. Be gentle. Avoid bending the piece of cardboard between the slits.
Do the same on the other side. (Thin rubber bands work best because they fit into the slits without bending the cardboard too much.)
Tip: If the slits bend open… If the rubber band comes out of the slits because they bend open, tape the slits in place while the rubber band is wrapped around them. It’s okay to tape over the rubber band.

Assemble the blaster.  Slide the plunger into the larger tube (called the “grip”). Power up. Hook each rubber band around a pencil end.

Load a ping pong ball. It should rest on top of the plunger.
Tip: If the marshmallow falls into the plunger… If the plunger is so wide that a marshmallow falls into it, cut the plunger open, squeeze it narrower, and re-tape it.

Hold the blaster’s outer tube.
Pull the pencil back to stretch the rubber bands. The ball will drop into the blaster.
Release the plunger. Watch your ping pong blast across the room!

Why is works:
Ever jump on a trampoline? When you push the trampoline’s surface down, you store energy in the springs. Stored energy is called potential energy. You go flying back up when the stored energy changes to motion energy (called kinetic energy).  In your slingshot, pulling the plunger back increases the rubber band’s potential energy. Let go, and the potential energy turns into kinetic energy—the ball goes flying. Bows-and-arrows and shock absorbers on a bike use potential and kinetic energy to work.

And that is that - go play.