Much more fun than shooting tennis balls out of tin cans taped together. There is a bit of a problem with the description however which is that while air speed will be limited to the mach number that is the net air speed. Generally in a projectile weapon the accelerating gas is moving along with the projectile and so within the moving gas cloud the projectile can much much faster than the surrounding air. That is why your typical 30-30 hunting rifle will have a muzzle velocity >2000 fps which is in excess of the mach number (and the speed of sound).
But you're limited by the mach speed of the fluid pushing the object, not the mach speed of the fluid you're traveling through. A bullet out of a rifle is limited by the mach speed of the hot gases created during combustion. A compressed-air gun, however, is limited to the speed of sound in compressed air.
This is a rediculously unsafe project. Plastic PVC is not rated to be able to contain that much pressure (better to have used metal pipe and brass hardware) and hydrogen gas is extremely flammable.
PVC also doesn't show up well on x-rays either. A student at our high school had a potato cannon explode next to his hip one year. He ended up with a trip to the emergency room where the radiology tech shared that nugget of wisdom with him. Fortunately, when PVC fractures, most of the pieces remain large, so he was mostly bruised, but he had a couple of nasty cuts.
In this case, however:
> Fortunately, everyone was safely behind a barrier a safe distance away, and no one was hurt.
It sounds like this particular experiment was conducted by people with enough sense to stay out of harms way.
Great experiment, great writeup, great parenthood.
Extrapluskudo's to your mom for making you figure out what went wrong. That's the way to teach something. Build->Fail->Rinse->Repeat
My Mom actually made me collect all of the pieces, lay them out, and try to root-cause the failure.