Make special ammunition for any weapon by selecting the weapon and using
. The only special ammunition you can craft at the beginning is fire arrows, but they count. You learn to make other types through skill training and story progression.
There are a couple of peculiar things about the way this achievement is tracked. The first thing to know is that all of your 50 special ammos must apparently be crafted in the same save file. I got to 96% with this achievement on my first game, then started a new game on a new save file and the tracker never moved. I started a third game, and it still didn't move. I went back to my original game, crafted 2 more special ammos, and the tracker moved and the achievement unlocked.
The second thing is, I've noticed that many of the cumulative achievements for this game don't track when you're replaying from a checkpoint after a death in Deadly Obsession difficulty. You can still get the achievement, but it will take longer because only the special ammo that you craft on your first playthrough of each part will count. I cannot confirm that this specific achievement works this way, but so many of the other ones do, I assume this one does also.