pf2e.fyi
From Pathfinder Guns & Gears, which isn't enabled.

Piercing Critical

FEAT 18

Your attacks with your preferred weapons deal especially devastating wounds. When you Strike with a firearm or crossbow for which you have legendary proficiency, you critically succeed if you roll a 19 on the die, as long as that result is a success.

Pathfinder Guns & GearsORC