I once cast pistol bullets from a mystery "linotype" that I later decided was probably babbitt. They cast well, and I water dropped them as well. I wanted really hard bullets and I got them. I cast them for a 40 S&W, and 45 acp. I sized and lubed them, and loaded them with a pretty solid taper crimp. I remember hearing a little creaking sound when I taper crimped them, but thought nothing of it at the time. As the bullets fed into the pistol, I often felt something hit my hand ever so lightly. The bullet holes in paper targets were perfectly round, and recoil seemed lighter than other loads I'd worked up with these bullets. I wrote it off to the bullets being lighter because of the alloy. I was wrong.
It turned out that the bullet noses were most likely fracturing as I taper crimped them. The feeling of something hitting my hand was actually the bullet nose coming off the bullet in the feeding cycle and being launched out of the pistol, and some of them would hit my hand during reentry. I've often joked about inventing the discarding bullet nose wadcutter for semi-auto pistols. I discovered the truth when I accidentally dropped a round while loading and the nose fell off.
You could simply let your bullets cool in the mould a few seconds longer, then drop them into deeper cold water to harden. You have a nice large crystalline structure showing, and I suspect you probably have a higher antimony content in that alloy than you're expecting.