Which type of black powder is used only in priming pans?

The correct choice is the type of black powder known as FFFFg. This grade of black powder is specifically formulated for applications requiring very fine granules, which is ideal for use in priming pans of flintlock firearms. The fineness of FFFFg allows it to ignite quickly and reliably, ensuring a rapid spark when struck by the frizzen in the firing mechanism.

In contrast, the other grades—Fg, FFg, and FFFg—are coarser and are designed for use in different applications, primarily as propellant grains for firearms. Fg is the coarsest, suitable for larger caliber weapons, while FFg is used for medium caliber firearms, and FFFg serves smaller caliber weapons. Thus, FFFFg stands out because its specific fineness is tailored for initiating ignition in firearms that utilize a priming pan mechanism.