I've regularly used Goex FFFg in my .50cal w/o any issues, but would not recommend it for larger bores.
Drop any FFg charge (or recommended charge) 20%, when switching from most BP FFg's to most BP FFFg - IOW, if you were shooting 100gr FFg, make it 80gr of FFFg.
(To simplify things, I use FFFg as both the main charge and the pan/priming charge in my .50 rocklock, too.)
If you're not already using Swiss BP, and Swiss FFFg is used, ANOTHER 10% drop in the powder charge (10% from Goex/etc FFFg, 30% from FFg) is mandatory.
Swiss BP is made differently than other BP's like Goex/etc, and it burns more efficiently. (It's more costly, also)
BTW - Your topic referred to "weights" of BP charges. Never actually weigh, as on/with a scale, black podwer charges.
Only use a volumetric powder measure to measure the charge size.
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