Yes that is it - in my opinion the very best antique projectile you have ever found or likely to find. It is the projectile from a very early Adams revolver ‘Dustbin Cartridge’ - it dates to the American Civil War and the Maori Wars and was made for the Model 1851 Adams and 1854 Adams revolvers.
Some references for you which I found last night
A complete one at $200 US or about $300 NZ - note that this dealer has never had a complete one
Note the price being asked for this well weathered one $65 US or $100 NZ
The guns below are the specific revolvers that they were made for -
I have ever only seen one or maybe two in the collection of an old guy and other than his I have never seen another so that is how scarce it is. A great find, Treasure it as you are not likely to find another. Your very best projectile.