PVC Air cannon

[CEPenworks]

So my son wants to build an air cannon out of PVC for a scholastic's project. They have a 1" rubber ball they need to shoot at a target. I have some 4" sch40 laying around and thought that would be great for the pressure reservoir. We checked and 1" PVC seems good for the barrel. Problem is finding fittings to get from 4" to 1" sch40. I thought this was going to be an easy project.

[Stwood_]

Careful with that. Some have used pvc as shop air lines. and have over pressured it and it's shrapnel if it blows.
I have not experienced this myself.

[crokett™]

We made an air cannon out of PVC. We dialed down the pressure on the compressor so that it would only pressurize the cannon to a certain amount. I think we had a 3" reservoir. Here's one way to get from 4" to 1"

https://www.homedepot.com/p/DURA-4-in-x-.../203225035

[JosephP]

What are you using for the valve?

I made one using a sprinkler valve.  Works like a charm.  Ball valve will be the cheapest/easiest, but is slow opening and hard to hold steady.  A butterfly valve like these is probably the in between choice

One thing I did learn...most instructions have a pretty big tank.  That is not necessary...a pretty small tank will do the job.  A bigger tank just keeps letting out more air after the projectile is long gone.

TreeStuff markets a pretty nice design that would not be difficult to duplicate (even with PVC).  I made mine more U-shaped...I like their design with the handle

[JosephP]

We made an air cannon out of PVC.  We dialed down the pressure on the compressor so that it would only pressurize the cannon to a certain amount.    I think we had a 3" reservoir.  Here's one way to get from 4" to 1"

https://www.homedepot.com/p/DURA-4-in-x-.../203225035

I put a 65psi blowout valve on mine.  My barrel is 2.5" PVC - it is as if somebody designed tennis balls to fit in this size!  I filled one with BBs.  at 65 psi, it shoots a 16oz tennis ball 90' straight up.  I only used a 1" valve.  When I was building it probably 10-12 years ago, there was a video of a guy that used a 2 or 3" valve...he shoot tennis balls through 3/4" OSB.

The Forest Products lab in Madision, WI has a 2x4 launcher that they use to test tornado-worthiness of different building methods. 
They used PVC...some pretty sharp engineers working on those projects...I figured if they were comfortable with it, I would use PVC as well.   A quick search didn't find that video, but here is a similar operation:

[CEPenworks]

I finally found al the pieces. Picking through what is DWV and what is sch40 gets tedious. So one pump with my bike floor pump was too much for what he had to do. We cut off the 4" and went with 1" reservoir so we could actually put some pressure in it. Problem was we could not get a consistent 4m target. We are using a PVC valve. Possibly too much variation in the ability to turn. We plan to put the 4" back on and have fun shooting things. For the school project we are going the ballista route. That one I can do with wood scrap.

[MstrCarpenter]

I made one with my son. It utilizes a "dump valve" design. The 5' x 1-1/2" PVC barrel extends through a 4" x 1-1/2" reducer into the 4' x 4" PVC tank almost to the bottom that is fitted with an end clean-out plug. Air flowing into the plug pushes a plastic disk against the barrel while air leaking past fills the tank. Stop filling and the higher pressure in the tank instantly pushes the disk back and the air is dumped into the barrel. First potato smashed apart on a tree about 50' away. Second went about 600' before we lost sight of it; it was still climbing! Now I have to find the picture my mother shooting it. 4'-10", <100 pounds soaking wet, all dressed up for Easter, including the hat... at 85 yrs. old.