Up to 5-20% Strength based on the charge time of your heavy attacks. Stacks 3 times.
3.5-5% Strength for 3.5s on Hit. Stacks 5 times.
1-4% Toughness for every 10% of magazine spent during continuous fire. Stacks 5 times.
4.25-5% Strength for every Enemy you Stagger. Stacks 5 times.