ref: Wrecker -where's the Gunners ?/ Tampions:
The tampions were the bits that were inserted into the barrels of the guns and screwed up tight (so they didn't fall out), to stop the ingress of water, goffers to you and me. The shiny bits with the ships crest on were simply known as muzzle covers, which fitted over the actual barrel. these were removed at sea.
In the case of roughers at sea, the tampions could also be covered by 'rubber johnnies' which fitted over the barrels when the shiny bits were removed.
The rubber johnnies were also used to ensure Gastight integrity (until the first shell was fired), obviously, the tampions were removed before-hand.