Alchemy 71 synth 13 smithing Fantastic synthesis to finish recapping alchemy sub with. I received a .1 about every 3 synths which is pretty stellar in my book.
Maybe I'm wrong, but I used these a ton when I was upgrading my relic and there doesn't seem to be a "stun rate" at all. The stun can only proc once per a certain amount of time (10-20 seconds?). Can test this yourself if you don't believe me. Go shoot something with one of these bullets, the first bullet will stun 100%, the second bullet will not unless you wait long enough before firing.
Therefore, the "stun rate" is 100% if you wait long enough between shots. If you don't, then it will be less.