IMO, It doesn't look like a cartridge case from the picture. There doesn't appear to be a primer and it doesn't look like a rimfire cartridge. It also doesn't appear to have a rim or a place for the extractor to catch the cartridge. It looks like some sort of cap. Sorry I can't help more.
Not a shell casing as I cannot see any primer at the end. Looks to be a little rusty. A brass cartridge would not look like that. Brass simetimes turns a greensish color.