I agree with your description Tom, and come to think of it a 'Drill' (dry practice use only) would have a mid-blue coloured ballistic cap, at least in UK forces. What makes me think it's probably inert, or 'empty', is the lack of the slightly protruding end flange at the 'open' end, but then I have little experience of artillery rounds, only AT guns and mortars. On British practice rounds, there is normally a long recesses channel on at least one side of the main sleeve,often painted red, rather than holes.