Yesterday I cut various threads on the lathe attempting to get something that would fit the existing nipple seat. It turns out that what I took for a tapered thread is basically M9x1 thread covering 60% of the length, then M8x1 covering the rest. M9 thread is worn on the top more than on the bottom so the whole thing looks like a tapered thread.

I suspect the original thread was M9x1 and the bottom 30% of the nipple seat was not threaded. Then someone came and threaded that hole, then made a nipple that is held by only threads at the very bottom of the seat.

A normal M9x1 thread is held properly despite the wear visible. I'm considering making a sleeve that would screw into that M9 thread, with a hole threaded with M6x1 into which a standard musket cap nipple could be screwed it. When making that sleeve I'll probably try to make its lower portion go into the M8 thread too so it is held very securely.

I also measured the bore diameter properly this time and it measures 20.7mm or 0.815 so half a millimetre too tight for an 8 bore. I have mixed feelings regarding this. On one side it is good that the measurement is closer to the original calibre stamp of 20.4mm as it means the barrel is probably close to its original strength. On the other side it is a bit of a disappointment that I can't think of the gun as "real 8 bore".

Edit: It turns out metric musket nipples have M8x1 or M8x1.25 threads which is a bit too big to sleeve in M9x1. A tiny revolver nipple would look silly on this gun. Probably making a nipple that fits into the existing M9 thread will be easier.