Sabots for the shell and a gromet wad over the shot.
"Ordnance Instructions for the United States Navy." by Bureau of Ordnance, USN
These boys were called gromets.
"How Britannia Came to Rule the Waves" by W.H.G. Kingston
An old word for gromet, or apprentice.
"The Sailor's Word-Book" by William Henry Smyth