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I did not put an internal cone on my liner for the express reasons of not vectoring pressure outward and not catching fouling. If I'm interpreting this issue right the trouble seems to be that the fouling builds up in layers on the flat face of the breech plug until it reaches a level to cover over the flash hole. When I picked it clear the fouling would yield to the pick intrusion and felt like a charged chamber of black powder.@M. De Land, there's a couple of things happening here. Your rifle is telling you that the touch hole and likely the internal cone is being blocked by fouling. What it is not telling you is the composition of the fouling. Is the fouling moist? Is the fouling crusty?
Your scraping of the breech face removed fouling from the bore but not from the internal cone.
The thorough cleaning of the bore and touch hole might help but the cleaning must include cleaning of the touch hole.
In a related thought, I do believe that @rich pierce is on to an interesting observation. With the touch hole much above 0.100 the breech face, a wiping jag will carry fouling into the touch hole and scrape some fouling bac into the internal cone. Not a lot but it will build up to become a block.
Picking moist fouling will do little good as the fouling will collapse back and block the touch hole. I am assuming you are using a solid pick to open the touch hole. Look for the largest of the dental flossing brushes or the airbrush set from Harbor Freight. The smallest brush fits through my touch hole and does a good job of cleaning the internal cone.
Airbrush Cleaning Brushes. 5 Pc. (harborfreight.com)
By around the 20th shot fouling will be building up and require a lot of touch hole and breech face maintenance to keep the flash channel open. When I use the smallest airbrush tip to open the cone, I will poke a few grains of 4f into the touch hole. Because the touch hole liner is internally coned, you won't have a fuse effect, but you will have a jet of flame blasting into the main charge. When my touch hole gets blocked, that's what I do.
I don't think the distance of the vent ahead of the flat breech plug face is the problem because all of my percussion rifles I've made have the nipple vent much farther ahead of the breech face than does this flint gun and I don't have the same trouble with them.
I think this is actually a good thing in the long run as my inkling is we are about to learn something about how flint guns operate that could prove interesting.