I never tried FFFg, but I can testify that FFg will NOT reliably ignite with #11 caps, even if you remove the nipple and prime the snail, in sub-freezing temps. I gave up at that point and went back to Pyrodex for sidelocks even though I hate the stuff after being spoiled by the easier cleanup of 777.

I have not had a gun where I could try musket caps so I have no information for you there. Musket caps + FFFg....MAYBE....