Guys,
The place that I use a tool like this is to give a tapered lead in at the breech end of a barrel that is to be rifled. The rifling head cutter requires some sort of a funnel shaped lead in to start the cutter. You really cannot start the cutter on the square edge of the gun bore. I am sure that you can envision the need for this. After the rifling is completed, this tapered lead in is cut away to put in the breech threads.
As observed, this one would be too coarse for precision barrel coning, at least when compared to the modern type of lapping tools used for barrel coning today. Maybe it was used somehow for a smoothbore fowling piece barrel muzzle treatment. Any other educated guesses out there? Remember that, just like today, old time gunsmiths were certainly called upon to make any type of small metal parts for their neighbors. So this tool may have had nothing at all to do with the gunsmith trade. Who knows?
Jim Everett