Thanks for the reply, MarkM.
You are right it is just about mating the threads. But I think you may have misread my question. You say, "measure your i.d. bore..." That is what I need to know how to do and what I was asking- how does a guy measure the major diameter of a hole less than 1" in diameter let alone determine the thread pitch and form?
Since this was a Belgian made turn of the previous century firearm the thread is unknown. It seems to use various threads. Again, since the bore is so small it is hard to get a read on the thread pitch and profile. One cannot see the thread pitch gauge perpendicular to the gauge. I have played around with it a bit and think I have the thread figured out but still have some niggling doubts.
The idea of a cerro safe casting would make that thread determination a lot easier. It would also make the major diameter of the threaded hole easier to measure.
In any case I would make up a plug and turn the threads a bit at a time, as you suggested, until the final fit is attained. Then I can measure with 3 wires and thread the barrel by sneaking up on it.
Gun threads are a weird mix. German built Mausewr action use a Whitworth thread. Many smithys just use a 60 degree thread and force them to fit. Seems a bit bush league to me. Many have weird thread pitches like 6-48 and some can be metric or just about any thread that was ever made. One thing that is certain is that you can't assume.