The Collaborative International Dictionary
Breech pin
Breech pin \Breech" pin`\, Breech screw \Breech" screw`\ . A strong iron or steel plug screwed into the breech of a musket or other firearm, to close the bottom of the bore.
Wiktionary
breech pin
n. A strong metal plug screwed into the breech of a musket or other firearm, to close the bottom of the bore.