got it back from the gunmaker, tig welded here and there and the nippel is out. have two now, one for small and one for musket primer and will use both.
maybe the old nippel will be going back, tig welded in place and serve as a bushing with a thread inside for the new nippels.

bore is smooth and smaller than I thought - only 6,9 mm.

still some work to do