Contrary to most of the folks here, I like to shoot Goex FFg instead of FFFg in my C&B's, especially the open-top styles. According to my sources, the old-timers considered 3F suitable for .31 and smaller revolvers.
2F will produce lower pressure (and velocity) and while the newer guns are made of stronger steel, I think the guns will last longer not pushed so hard. I GENERALLY get a bit better accuracy too, but 2F wants a little more compression than 3F, though not as much as Pyrodex.
With 3F, I just feel the ball crunch against the powder. I probably put 10-15 pounds more on 2F. Pyro wants about as much compression as you can put on it with a full length rammer.