RE: 4F Blackpowder--WTH is it?
As Charley said, use Goex FFFg to prime with. When I was getting all the stuff this year to set up my Hawkins flintlock I was shooting FFFg as the main charge and the priming charge which is very common in many places. That way they do not have to carry two different kinds of powders with them into the field. If you have a good flint you will not notice the difference all that much. Many of the large rifles and smoothbores with big pans and flints such as the Bess and some of the large other muskets use FFFg and some have even reported using FFg. One other shooter I know uses his flinter with FFFg as a priming powder all the time.
I was lucky enough to come across two pounds of the Goex FFFFg and bought both pounds of powder. Needless to say I have enough priming powder for many years to come.... Now if I can only learn this flint knapping. I have tried every procedure described on the forums and so far have not been able to do it..... Oh well live and learn.