I'm moving forward on the assumption that this is the standard T&T 149 -- a regular Union three-ringer. It has a cone cavity and a medium-sized skirt. What is a bit odd about this bullet is that the rings are rather thin. Usually, they are more pronounced. I know it is not a .54 machine-turned bullet (T&T 130). There's a chance this is a T&T 153.5. Either way, it's a tad different than the standard Federal .557/.58 minie.

 

This was dug near Monocacy, Md. on private property, with permission.