Protagonist:
I'm not a GM, but I would think that fanning a single-action revolver would fall under the same rules as firing a burst-fire-capable semiautomatic pistol (
i.e., a Savalette Guardian) in burst-fire mode.
That is, it'd be faster than normal, but less accurate.
At very close range,
i.e. pointblank to short, it would be devastating, but only against a single target.
That stuff in the movies about a guy taking down three or more adversaries in that fashion just wouldn't happen.
IRL, fanning only wastes ammunition, unless you're firing at something incapable of shooting back.
For example, check out this Website:
Bob Munden: The Fastest Gun AliveThe gentleman in question can fire 5 rounds from a Colt single-action in under one-quarter of a second.
--Foreigner