I also loved the video. But I'd think if one wanted to deploy "simple" mechanical projectile weapons at scale, pneumatic launchers would be easier, more compact and more reliable (way less cool-looking and operating, though!).
I helped design the "reloadable" part of a golf-ball shooter that used acetylene as the accelerant. I don't think it went supersonic, though. There's videos (embarassing ones) on youtube. He eventually designed one that can shoot full cans of monster energy hundreds of yards.
the reason i bring it up is we used to design and build pneumatic ones, that could send C and D batteries thousands of feet if you arced it right. that was with ~125psi, we always wanted to build out to 200+ psi, but instead went into accelerant-based cannons and he got a job doing machine drawing with autocad. the rest is history.
edit: https://nextcloud.projectftm.com/index.php/s/cobra5x we show off the part i designed, toward the end on the "reload" - he designed and built the entire thing.
Really not trying to call you out here, as a teenager I once had the idea to load up my marble cannon with dead AAs (they're free!) and got a "do you really think that's a good idea?" from an environmentally conscious adult when I went asking for dead batteries.
Don't get me wrong, oranges are dangerous too, coming out of the sky at terminal velocity, but at least when they hit the ground they disappeared into a fine, 3 meter wide mist.
He got an efficiency over 40% (if memory serves, I watched this video yesterday) -- I wonder between pumping and discharge how efficient a pneumatic system would be, but I can't imagine that it's higher.
How do you imagine making the cycle time better "by a very wide margin" compared to lifting 100 pounds less than 10 feet?
But yeah, the fact that it's super cool looking - and sounding - is a key factor in my willingness to play with one too, one day. ;)