How to make a water balloon cannon

Summer is coming soon, and that means LOTS of back yard time for my kids. We have a zip line, climbing wall, trampoline, swing set and a pool. I always am looking for ways to make the space that we have more fun and engaging for the kids, which led me (obviously) to making a cannon/turret for water balloons.
My kids are young and of course I wouldn’t want to make anything powerful enough for them to really hurt each other so I decided against using surgical tubing or anything like a water balloon slingshot, but instead made a cannon, or more of a mortar I guess that is powered by their own arm strength.  How far they can pull it back decides how far it can travel.  I made the weapon able to pivot side to side, but not up and down as you stop them from targeting each other too directly (3 boys who have no concept of how much things actually hurt).

So check it out, build/modify it at your own risk and most of all, have fun!

I used a 3′ length of 4″ and 1″ PVC pipe.

I also used a 3″ cap which fits right inside the large pipe.

I traced the smaller pipe on the larger one, marking it a little larger than real life.

I marked a cross line several inches from one end.

I ran it through the bandsaw which cuts the line on the top and bottom of the pipe, to cut both lines.

I used a cut off wheel on a Dremel to trim the piece out.

I switched the wheel out for a grinding stone and cleaned off the corners of the cut. This is to cut down on friction.

I made sure the smaller pipe fit in the slot and moved freely.

Then I made sure there was enough space on each side for a hand, and marked the pipe.

I cut the small pipe along that mark then rounded over the ends on the disc sander.

With the cap, I used the belt sander to carve out a small dip in the  top.

Then back to the disc sander with the cap. I sanded down the entire outside  of the cap to make it’s diameter a little smaller.

When you cut the section from the PVC, it puckers a little, making the interior size a little smaller. Sanding the cap makes it fit better inside.

I drilled a small hole in the center of the cap. Drilling from the inside of the cap helps drive the bit to the center. I also drilled a hole in the middle of the smaller pipe section.

I ran a long bolt through a large washer, then through the cap from the inside.

I added the small pipe to the bolt and added a washer and nut to hold the pieces together.

This assembly dropped down into the slot cut in the larger pipe. Then I added some PVC cement to the open end and slid on a 4″ coupler.

Since the pipe had the pucker, I clamped the cut edges to the coupler while the cement set up. This helped maintain the correct interior shape.

I cut two pieces of old pressure treated 2×4 into sections about the same length as the pipe.

I drilled a hole through the coupler into the 2×4 below.

I connected them with a short decking screw.

I used some polypropylene strapping to hole the pipe down to the 2×4 on the front and back.

These straps cut very easily with a utility knife, but will hold up well outdoors and are not sharp like the similar metal strapping is.

I also had some used pressure treated plywood that I cut down to a smaller size, with one diagonal side.

I traced it and cut a second identical piece.

I screwed these to the sides of the 2×4 with decking screws. This gave lift to the front of the cannon.

I cut down two more short sections of 2×4 and screwed them onto the front edge of the side panels.

I used 4 decking screws each as these will receive a lot of force when the cannon is used.

On each of the 2x4s I drilled a hole and screwed in an eyelet. This was in line with the cut out in the pipe.

I carefully cut the hooks off of a 4′ long bungee cord.

I cut the cord in half and burned the ends with a lighter to stop them from fraying.

I fed the cord through each eye and tied them into a figure 8 loop knot. The other end of this knot can easily slip over the handle piece.

I adjusted both cords until they had mostly even tension on both sides.

The entire thing got sprayed black, which made it really look like a cannon.

On our playset, I drilled a hole in the railing.

I also drilled a hole roughly in the middle of the second 2×4.

I added two washers on the railing then ran a washer and bolt through the 2×4.

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