One other thing to ensure there's no misunderstanding about statements regarding powder to be used for priming...priming powder (4F) is designed/sold for the purpose of priming. For example, Goex 4F is all I've ever used since I've been into Flintlocks and it's always been 100% reliable...never a hint of ignition problems.
And that's covering range use and year round hunting under all weather conditions...including intentionally going deer hunting when it was raining and/or being caught in rain while hunting...3 times for deer and 1 time for turkey that's happened and killed deer/turkey on those trips without hesitation on the part of my Flintlocks.
One of the nice challenges of using a Flintlock, particularly hunting with a Flintlock, is to learn to manage it's environment properly, mainly as it relates to ignition...and regardless of the granulation used for priming while its raining, you simply need to do a couple very simple things:
1) Keep the muzzle down lower than the lock so water can't run back alongside the barrel & barrel bed into the pan or vent;
2) Keep the lock up under the coat or rain gear where it’s dry;
3) Refresh the prime periodically...costs a penny and you'll have very fast, 100% reliable 4F ignition every time in spite of the weather;