This is a really cool experiment by Taofledermaus where he makes some water-filled hollowpoints. The expansion is intense.
This whole deal needs to be kicked up a kick, with better slo-mo and real ballistics gel. It would be interesting to see how hydraulic ammo would do through layers, and also the performance of centerfire cartridges modified like this.
Who knows, this could change cartridge design. Keep your powder damp.