Further clairification for those who don't want to go read what I said:
As I mentioned in that topic, the video was made primarily for the people shooting those new fangled guns that we don't talk about.
Almost all of the people using them, to a man use heavy, elongated bullets and IMO, 3Fg and 4Fg powders will create tremendous breech pressures.
I'm talking about pressures that are getting up into the smokeless powder range.
Because some of us Traditional rifle shooters also shoot some heavy conical's in our sidelock guns, I think the statement about not using those powder grades is as valid for us as it is for those "other guys", but only if a conical is being shot.
For the people shooting patched roundballs, the 3Fg powder will work fine.
I notice that in the "Lyman Black Powder Handbook & Loading Manual", they only show 2Fg, Pyrodex RS and Pyrodex Select powders for the heavy conical's they tested. No 3Fg loads at all are shown.
I wouldn't recommend 4Fg powder for anything other than flintlock pan priming.
I suppose it is HC for the little .31 caliber cap & ball revolver but even then, it would be for a light 8-10 grain load of the stuff.