Make molds with gap sealer silicon mixed with cornflour makes a putty B4 it sets.

Maybe this, wat put into Mold.
But it shrinks a bit I think.

Here's make Mold.

I was having a bit of a think about this idea and it seems it could be greatly simplified and also made more efficient: https://i.imgur.com/Csm81PX.gif The piston itself becomes the hammer that opens an inline balanced valve. Sorta thinking out loud. Piston be generating air infront of it, as heade...

If you have the same air volume and the chamber is 1/3 the volume, the pressure will increase by 3x. https://www.omnicalculator.com/physics/boyles-law Interesting to note. Apllying my reasoning, punch into calculator. As cylinder reduces in volume. increases the pressure to compensate for The loss ...

How would hollow cylinder and sabot go.
Someone's thought about it.

Maybe Just do a vacuum cannon.
Be simplest.
Plunger, Aluminium foil for burst disk.
Use a ping pong ball or tennis ball.
Be more in the "high school" theme.
Safer for demonstration.
Just my two bobs.