Something along the lines of a critical hit where it would trigger a stun/tread destroy or by triggering an ability. Alternatively, add a timed upgrade that would buff reload/accuracy or something to that effect.
OK, let's start with the easier one.
1. Reload/Accuracy buff
For this, you don't need any ability. To your requirement_action add to the action the according modifier.
2. Triggering criticals
Here I would make it switch weapons. The switched weapon would have under damage > on_penetrated_action your rand action that would have the criticals in it. You could do this also to deflection > on_deflected_action.
I hope this gives you an idea of how you could approach in your modding.