Pretty neat little example of a gas gun. If they used He or H2 for the pressurant, they could probably get M > 2. Wish they'd show some high speed video, I bet that ball has some interesting aerodynamics at M>1 (like the ball surface oscillating as the bow shock deforms the surface).
The next trick is to form the "ball" into a projectile with its own conical diffuser, expander, and nozzle section - like a ramjet centerbody. Then let it enter a second tube into which a premixed combustible mixture has been injected. Voila, ram accelerator, with exit velocity up to Mach 12 or so...
