I can't find the actual rules for the Volatile weapon trait anywhere. It's on the Concussion Mace in Daemon Hunters, but I can't find it in that book, or any of my other ones. I am just missing it or is it in some other book?

Can't find that one anywhere, the closets I got is from Rogue Trader:

Unstable:

Weapons with this quality use ammunition that is both volatile and unstable and can react unpredictably when detonated. When an Unstable weapon scores a hit, roll 1d10. On a score of 1 it inflicts only half Damage, on a score of 2–9 it deals normal Damage, and on a score of 10 it inflicts twice the normal Damage.

Given the description of the concussion mace, its not entirely off…

Yea, thats in the Dark Heresy core rulebook as well. I may talk with the rest of my group and see if we wanna just use that. It makes sense given the name and what it's on

Check in Deathwatch or Lathe Worlds I think. Weapons with volatile auto confirm righteous fury.