Here's a pic and diagram for a breech plug tool for old style screw-in breech plugs with the 2 holes for a spanner type tool, contributed by member Sprint.
The cap screw threads into the nipple hole and is snugged up. A couple smacks
with a plastic hammer on the end and a 7/8" wrench on the tool usually
works. Two hardened pins for the spanner function and a ¼"-28 cap screw that
will not bottom out in the nipple hole works good.
Tim