breech pin

from The Century Dictionary.

• noun In gunnery, a mounted plug screwed into the rear end of the barrel of a firearm.

from the GNU version of the Collaborative International Dictionary of English.

• A strong iron or steel plug screwed into the breech of a musket or other firearm, to close the bottom of the bore.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A strong metal plug screwed into the breech of a musket or other firearm, to close the bottom of the bore.