If you are shooting a chambered Breech rifle or pistol, consider using the finer grained Pyrodex P, instead of the coarser Pyrodex RS. The finer grained powder flows better through the small hole in the back of the powder chamber, into the flash channel, and over under the nipple.
Depending on which substitute powder you choose`, you may have to use "magnum #11 caps, instead of the standard size #11 cap. There should never be a need to use Musket nipples or Musket Caps with these powders.
Don't Believe me? Have someone fire off a cap on your gun at night, after you allow your pupils to dilate sufficiently to see in the dark. Keep your eye on the muzzle of the gun, regardless of length of barrel. Even Standard caps will send a 6" long flame out the muzzle. The Magnum caps will go double that amount- so I don't really see the need for them, unless you are trying to ignite coarse substitute powders.
Musket caps don't put out any more flame than the magnum caps, so spending the money for a musket nipple, and then buying the much more expensive musket caps is just throwing your money away, IMHO.
Unlike flintlocks, percussion guns seem to work best with tightly packed BP. I mean, you get the lowest Standard Deviation in Velocity with the powder compacted. FFFg Goex, or the comparable Swiss powder, and Pyrodex P will pack better than the coarser FFg Goex, etc. powders do. Remember, that a percussion rifle will INJECT its flame into the powder charge, burning a hole right through the center of it, for fast, and complete ignition.
Flintlocks can't burn powder that way, however, as its impossible for a flintlock to INJECT flame into the powder charge in the barrel. Flintlocks ignite the powder in the barrel more like lighting a fuse. :hmm: