Pain Points

1. Rank 0On Weak Point hits with Primary Fire:+10% Weak Point Damage for 5s. Stacks up to 10x.
2. Rank 1On Weak Point hits with Primary Fire:+20% Weak Point Damage for 5s. Stacks up to 10x.
3. Rank 2On Weak Point hits with Primary Fire:+30% Weak Point Damage for 5s. Stacks up to 10x.
4. Rank 3On Weak Point hits with Primary Fire:+40% Weak Point Damage for 5s. Stacks up to 10x.
5. Rank 4On Weak Point hits with Primary Fire:+50% Weak Point Damage for 5s. Stacks up to 10x.
6. Rank 5On Weak Point hits with Primary Fire:+60% Weak Point Damage for 5s. Stacks up to 10x.

Facts based on the Warframe Wiki (CC BY-NC-SA 3.0)