I don't know why you guys have to be so harsh, sheesh.
There isn't much comprehensive documentation about the new features, although one could argue that discovering how things work is part of what makes the game interesting.
Anyways, like Dervis mentioned, when you initiate chainfire, you shoot one less bullet than in a normal burst. However, after that, if there are still enemies visible, you can fire a 'warming' burst - which has the normal number of bullets - you are automatically in 'targeting mode' without having to press fire again if this is the case (and you can press Esc to cancel, allowing you to move/fire normally/initiate chainfire from step one). If you fire that 'warming' shot, and there are still visible enemies, you now fire 'full' bursts - the normal number of bullets +2. So you have a slower start but higher rate of fire in the long run, as long as you have ammo and as long as enemies are visible. However, if you switch targets and the new target's angle of fire is far different from your original target, some of your bullets will be wasted sweeping to your new target, unlike normal fire.
I suggest that chainfire be used when facing the capital B's or worse - enemies as tough or tougher than Hell Knights/Barons, and if your new target is at a different angle, restart chainfire rather than allow bullets to be wasted with the sweep.
