You will definitely want real Black Powder for the flinter. The substitutes are just too hard to light, and don' t give reliable ignition in a flinter. For a 50 cal you could use either FFg (2F) or FFFg (3F) for the main charge. Traditionally FFg is recommended for 50s, but FFFg is OK. It' s a little hotter, so you can reduce charges a little. For the priming charge you want FFFFg (4F). In a pinch you could use FFFg for both main charge and prime, but FFFFg prime will give quicker, more reliable ignition.

As for brand, any of the availble brands should give satifactory results. In my opinion Swiss is the highest quality available, in terms of velocity and consitency, followed by Goex and Elephant, in that order. Although, in my recent experience in chronograph testing current lots of Goex and Elephant for velocity and consistency, they have been roughly equivalent, with some lots of Elephant performing better than some lots of Goex and some lots of Goex better than some lots of Elephant.