I found 777 not to put much if any of a crud ring in my sidelock, I think it has to do with the cooler (11 cap) ignition.

The problem I did have was hang-fires under around 30F and no-fires below 15F. The caps were just not hot enough for it, though I'm sure musket caps would work fine.

I'd just use pyrodex, I always used ffG (RS) but I don't think it matters much.