Up to 5-20% Strength based on the charge time of your heavy attacks. Stacks 3 times.
1-4% to Instakill human-sized enemies on Chained Heavy Hit. Stacks 5 times. Note that other potential triggers will not be activated on Instakill.
5-8% Strength for every 10% of magazine spent during continuous fire. Stacks 5 times.
2-5 bullets loaded from Reserve on Critical Hit.