I've not built one of these, but it might be a way of administering fouling control and cleaning to the ante-chamber of a patent breech plug.
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Add a .30 cal brush to the front end of a fouling/cleaning jag (drill and tap). Cut an "X" in the middle of a cleaning patch so it can be pushed over the brush as the bore patch. Then drape another patch over the brush. Wet the patches, run down the tube, and clean both the main and ante-chamber at the same time.
Another thought, while building this tool, is to either fill the jag's cup with silver solder or grind it flat. A cleaning jag with a concave face makes no sense at all since it won't effect cleaning of the breech face. This is even more true for classic flat faced onshore breech plugs.
At least that's the concept.