It's a necked down rim fire cartridge, which someone surely will know what it is. I've found that really old dug cartridges, the ones that used copper cases loaded with black powder and a lead bullet, must have electrolysis take place between the copper and lead metals, or perhaps something that was used in the lead back then, like antimony to harden the bullet, what ever it is, causes the lead to degrade and crumble. Chemically I don't really know why, it's just something I've found when recovering early loaded 45-70 cartridges. It's an interesting find, and I'm also wanting to know what cartridge it is -- I'm wildly guessing it's an early Sharps cartridge.