With the greatest respect, I'll relate here that the methodology of using whatever grains of BP under the nipple in the case of a misfire or failure to load a charge in the barrel has been around pretty much since the advent of caplocks as far as I know, and certainly for the past 65 years I've been involved in muzzleloading. The pressure created by doing this is nothing as compared to the pressure created by a typical ML charge going off. The same should be true with a few grains of Triple7 or the like. But not, of course a few grains of smokeless powder. Don't go that route.
A well-loaded caplock ML rifle with a proper flash channel will has some of its charge right there, immediately beneath the nipple's base. It will be right there every time with most any ML shotgun.
Notably there is quite a bit of flash right before your eyes when a flintlock goes off.
And it is very seldom that a weak spot in a hammer occurs. What does happen at times is that someone puts a nipple in an old or new ML that is the wrong size yet manages to fit the threads enough to screw into place. Bad trouble waiting to happen as far as that nipple being blown out upon firing that gun.
Aloha, Ka'imiloa
