2F powder may eliminate the need to cover the flash hole.After making the cartridges I read on another forum some guys use a small square of cloth medical “micropore” tape to to cover the flash hole on the cartridge. I picked some up at Walmart (generic Equate brand called “cloth tape” in the bandaid aisle) and removed the scotch tape and placed a little square of the micropore tape over each hole.
Works great! The musket cap easily punched through and I had no hangfire at all, instant ignition.
In the day the gutta-percha cases in use would have been sealed with something other than tape. Perhaps a drop of melted bees wax.