The Collaborative International Dictionary
Breech screw
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 screw
n. A breech pin.
Usage examples of "breech screw".
Firstly, this breech screw must be slackened off to relieve the pressure on the vent piece.