Let me think about your touch hole liner being 5 mm. Most of the liners I am aware of are 1/4" which is close to 6mm. This would imply that the internal cone is much smaller than what I normally see. That means less powder is nest to the touch hole and pan. Now that you are getting reliable ignition when you fill the pan, I don't recommend that you replace the touch hole liner with a larger one. Anyone who can drill a clearance/flushing hole through a 5 mm (0.200" approx) screw should be able to replace the touch hole liner at some time in the future. Note that many reliable touch holes are simply a 1/16" to 5/64" hole drilled through the side of the barrel with no internal coning. Nice barrel flushing system. Now to get the barrel and all the channels dry so fouling isn't going to accumulate and block heat from the pan getting to the main charge.I put a wrench on the breech plug, it's not coming out easy so I'm just going to leave it.
Removed the barrel and gave the bore a good hot soapy mop, then put a soapy patch on a .30 brush and cleaned the chamber, then flushed lots of hot water both ways. I drilled out a 5mm screw that fits the hole for the touchhole screw so I can flush from the breech. Got couple small black chunks out, hopefully that is it.
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If it's working then continue on. A build up of fouling should be raising the impact as pressure increases. Concentrate on keeping a consistent hold and keep aiming until the smoke clears.