I have tried everything I could get my hands on over the last 55 years, My conclusions are real simple.
The best powder for side locks is Pyrodex. The best powder for inlines is Blackhorn 209.
The worst powders are the pellets they attract moisture and since those boxes are not seal they can start doing it right in the factory before they are shipped to the store.
_THIS IS FOR CLOSED IGNITIONS ONLY , OLD PLUNGER TYPES SHOULD NOT USE BLACKHORN.
I suggest a little test for you. Take your 777 and some Blackhorn out and shoot them so you know where the impact point is the take enough for 3 shots of each and put them in separate pans and set them in the yard over night then take them out and shoot them on the same target as before to see if the poi has changed. To do this you need a better designed BP than the normal CVA.
Speaking of which a BP with the flash hole burned out can cause your accuracy to be erratic with any powder it is the first thing to check when you have an accuracy problem.