Up to 5-20% Strength based on the charge time of your heavy attacks. Stacks 3 times.
60% Strength on your First Attack every 3.5-5 seconds.
5-8% Strength for every 10% of magazine spent during continuous fire. Stacks 5 times.
7-10% Strength for every 4 Single Target Hits. Lasts 2s and Stacks 5 times.