Cut one of the panels off a cardboard box with scissors to leave a raised cardboard base. A cereal box with the lid taped shut and one large panel removed, or a tissue box with the slotted panel cut off, is ideal. An upside-down shoe box with the lid removed is a strong alternative.
Use duct tape to secure a toilet roll tube to the top of the cardboard box base. Position it centrally, width-wise, across the box roughly two-thirds of the way along the length. Run four strips of duct tape at equal distances over the top of the tube with 2 or 3 inches of each strip stuck to the cardboard box on either side. Also secure the bottom of each end of the tube to the box using 3- or 4-inch strips of tape.
Tape the two popsicle sticks together with about 1 inch of each stick overlapping the other. Wrap plenty of duct tape around the sticks to make a strong join.
Take the middle out of a small matchbox. Tape it lengthwise on one end of the doubled length of popsicle stick. This is the ammunition bowl at the top end of the lever.
Make a hole through the top of the cardboard toilet roll tube with a skewer. Push it further on through the cardboard box base. Wriggle the skewer from side to side to make the holes wide enough for a popsicle stick. Turn the box over, if necessary, to widen the bottom hole.
Twist a rubber band into a double loop and slip it over the bottom of the popsicle stick lever. Move it up to the top half of the lever.
Push the bottom end of the popsicle stick lever down through the two holes. Leave the join in the sticks above the cardboard tube. Secure the stick in place underneath the cardboard box with duct tape. Strengthen the edges of the hole in the top of the cardboard tube with tape.
Position the rubber band loops over the taped join in the popsicle stick lever. Hold the lever fully back with one hand. Pull one of the rubber-band loops with the other hand, down the length of the box. The other loop will tighten around the lever. Make a mark in pen on the cardboard box where the rubber band reaches to when it is taut. Release the lever and band.
Make a hole at the pen mark with the skewer.
Stretch the band out again and push the end down through the newly made hole. Turn the box upside-down and push a toothpick halfway through the small rubber band loop underneath. This will hold the band in place.