I may be, and probably am a mile off here, but could it be launching shell for a ball or maybe anti zeppelin chain shot ?
I've read that the British navy were issued with the latter which may have needed to have been waterproof
Probably wrong but just a thought
Quick edit... I've just found a picture of the anti zeppelin shells but they are a full length brass cased shell. Not like the ones you have sorry.

Last edited by El Garro; 05/02/12 12:58 PM.
Rust never sleeps !