Hope you get the result you want, I can't wait to see it.
The pellet hitting water drop reminded me, I once saw a shooter hit a raindrop with his shot during a comp in heavy rain. It was about 8 feet from the muzzle. It looked like a tiny puff of smoke from an anti aircraft cannon in WW2 films. I was watching from about 30 yards away up a hill. It's something I've never seen before or since. This was at Guiseley FTC and no bugger else saw it, even the shooter! And they didn't believe me.
Catching that on a still or video sequence would be awesome.
Also, and much easier to capture, I've seen some impressive vapour puffs from the barrel ports of my Steyr LP10 on cold damp days.