Black Powder made in America( Goex)is coated with graphite to do several things, among which is to impede the absorption of water from the air. That is why the powders are marked with a small "g" after the size symbol- " F" . Its the residue, which exposes both the nitrates, and the sulphur in the powder to air, that quickly absorb moisture from the air, and create both nitric, and sulphuric, acids, that eat metal!
normally, unless the powder is dampened, either by water being left in the barrel before it is loaded, or by water getting into the powder charge after its loaded, The powder is good for months and years. After all, we have discovered cannon barrels at the bottom of the oceans that have been there for 300 years and more, loaded, and when the barrel is cleaned out, the powder will still burn.
I suspect that some kind of wood plug was put in the vent holes to prevent water from entering the powder charge, so that the barrels could be kept loaded during voyages, for instant use, but that is just a guess.
The best ways I have read here in prior posts to keep your powder dry is to put a piece of plastic wrap between the muzzle and your PRB when you load, so that the plastic acts as a moisture barrier between the powder and PRB. Plastic wrap can also be placed under a percussion cap, and for flintlocks, putting a patch soaked in alcohol, under the frizzen, in a bare pan, with half the patch lay up along side of the barrel and covering the touch hole seems to be a very good solution. The evaporating alcohol pulls moisture out of the main charge through the vent hole. Cover the whole action with a Calf's knee, and/or tuck the action in your armpit, or under a poncho, in the rain, and you should have dry powder. With a flintlock, you need to consider priming your gun when you see or hear game, or else you have to change the priming powder every 15-20 minutes or so. The alcohol in the patch will NOT foul the priming powder, and its best to use the coarser FFFg or even FFg powders to prime your rifle on rainy or foggy days, rather than using the smaller, FFFFg powder.
Do carry dry paper towels, or towels and dry cleaning patches with you in an inside pocket to use to wipe down your rifle frequently in the field. Keep powder dry was such a problem back in the days flintlocks were the only guns that battles were often " called off " when it began to rain hard, by agreement of the officers on both sides of a conflict. This practice continued right up through the Mexican American War in 1845, and only really ended with the American Civil war where Percussion rifles were the norm. However, the Civil War saw a few battles called because of rains because the powder used for cannons was too damp to be reliable. The first cartridge, Breech loading guns had been invented and existed in the 1850s, but were too small in caliber, and too few in number to be of much consideration at the beginning of the war. By the end of the war, the outcome of some battles depended on units using their own cartridge, repeating rifles, mostly Spencer rifles and carbines.
