You give us nothing for a size reference. If the spring channel is 3/16" to 1/4" wide it may be a primer holder for a percussion firearm. The rust inside is the remains of the spring that advanced the primers. But to be sure I'd need to see the "spout" to see if there is a way to hold and release the primers. The lower flat would have to be relieved for the nipple to slip into the primer.