I know its old but its part of the reason I'm asking. Firing line sounds good if I play with a team but that's not always the case; and I like this weapon. So something beside firing line. God I wish we could have 2 perks to switch between like some weapons.
Frenzy is 15% and requires you to be in battle for at least 12 seconds (this can also be accomplished by shooting 'immune' enemies).
Focused Fury is 20% but (on a magsize of 6) activates after 3 crits.
From a pure damage perspective FF falls behind early and will (after activation) require 9 more shots to go equal with Frenzy (12 total), and everything after that will be ahead of Frenzy.