This is a neat piece! After the Civil War the models of Spencer went to a smaller caliber bore size like this one , .52. This is one of those cartridges. We know this because on the bottom of the cartridge it has the large "U" stamp. This is for the Union Metallic Cartridge Company that didn't go into business until after the Civil War. The cartridge is the correct .52 caliber size for the model 1865 Spencer carbine or rifle. This one is in excellent non-excavated condition.