Doc, the page appears to use flash for the images. Judging from the photo of the bullet, they’re about 1/2 the height of an M-16 round, with much of the bullet being surrounded by what looks like a ceramic insulator. Presumably, that’s the propellant.
All the metals of the alkali group (the first collumn on the periodic table) will react violently with water. I remember the experiment you’re talking about, my chemistry teacher compared pottasium to sodium though. I believe cesium would have the effect that you’re shooting for. The reason they will NEVER produce these rounds is they’re such a fire/explosion hazard. If moisture ever got inside the round it would react with the cesium and set off the powder, not exactly something you could safely keep in your home.
I just don’t think exploding bullets are really necessary when you witness the damage a hollowpoint round can do.
The H&K G11 4.7mm caseless ammo rifle almost went into service with West German forces in the late 1980’s, but the Soviet collapse shelved those ideas.