If you are shooting a chambered Breech rifle, like you find with the T/C, Lyman, and CVA guns, or many of the imports, you need to use FFFg powder to flow through the tiny flash channel that leads from the back of the powder chamber to the TH. Do NOT pick the channel with anything, After its filled with powder from loading. Picking will simply move the powder away from the TH and make the gun more difficult to ignite the main charge of powder.
If you are shooting a flat faced Breechplug flintlock, with the normal TH on the side, you can shoot your choice of powders, FFg or FFFg. Picking the TH is for the purpose of exposing more granules of powder to the heat from the ignited priming charge in the pan. Clearing powder away from the mouth of the TH with the pick allows that ignition of multiple grains of powder to occur. An inside coned TH Liner, like the Chambers White Lightning liner, allows powder to stay near the TH, but for you to have a center HOLE in the powder across the width of the barrel's bore.
That allows multiple grains of powder to ignite, not just those near the TH. Igniting more grains is like touching off multiple fuses in the barrel. The powder charge in the barrel ignites faster than if you only light one grain(fuse) at the mouth of the TH.
The current state of manufacturer of breech plugs with powder chambers leaves very small holes and flash channels in these breeches, and the do not allow BP the Granular size of FFg powder to flow easily through those channels, UNLESS the channels Are Absolutely clean and dry, of oils, greases, and debris. Some of the imports seen have casting trash( dross) left in those narrow flash channels- as if the gun was being marketed as a wall-hanger!
A drill bit run through that flash channel will clean up the hole, make it round, and smooth it for shooting. Enlarge both the flash channel, and the hole entering the powder chamber and you eliminate the restrictions against using FFg powder in those guns, too.