breech closer,
breechblock a metal block in breech-loading firearms that is withdrawn to insert a cartridge and replaced to close the breech before firing
stopple,
stopper,
plug (bridge) a playing card with a value sufficiently high to insure taking a trick in a particular suit; "if my partner has a spade stopper I can bid no trump"
occlusion -
(meteorology) a composite front when colder air surrounds a mass of warm air and forces it aloft