This is a gas check - the driving band behind a shell - of a 17.72in (450mm) 100-ton coast defence gun from near Valetta, Malta. It was collected by my great-grandfather, Charles Valder (1860-1925), when he was a junior officer in the Royal Garrison Artillery, stationed with a coastal battery in Malta. The gun was one of four made Elswick in the 1880s for the British Army: two were emplaced in Malta, two in Gibraltar. This was the largest rifled muzzle-loading (RML) gun ever made, firing a 910kg shell - the Exocet of its day!
My grandmother, Charles Valder's daughter Isabella, told me that it was the result of a misfire during a practice shoot in the mid-1890s, when the family was stationed in Malta. (Normally, the gas check would have left the barrel with the shell.) They had it mounted as a dinner gong - and at 25lbs of copper it had an impressive ring. The mount made for it has long since disintegrated from woodworm. I saved it from being sold for scrap in 1970, when we turned out my grandparents' house in Sussex. I think it's probably one of the few pieces of this ammunition left in the world - they didn't have one in the 100-ton gun museum in Gibraltar when I visited.