This thing has been sitting in a draw on my tool bench for 25 years, and before that it was something my dad kept in his basement for another 30 years. I don't think it's a Civil War type, but I have no idea. Any musket bullet experts out there that could give me a clue? Thanks in advance.
It appears to be a "modernday casting" made by pouring molten lead into an original civil war "Ringtail Sharps" bulletmold -- or into a Reproduction mold. To see one from a Repro mold, go to the following webpage and scroll all the way down to the bottom for bullet #557489: Ol' Buffalo Bullet Mold Tables
Although original Sharps bulletmolds are very rare, some definitely still do exist. Many modernday blackpowder-rifle shooters like to use Original molds to make repro bullets. Your bullet is definitely a modernday casting, because none from the civil war era still show bright-shiny lead like we see in the grooves on your bullet -- unless it was kept in an ABSOLUTELY AIRTIGHT container for more than a century, which you indicate it wasn't. Still, the presence of that bullet in your dad's basement, apparently cast in an Original mold, indicates that just maybe the Original mold -- or a Repro one -- is somewhere in that basement.
40 or 50 years ago I had an original Sharps carbine that was still shoot-able, and I cast bullets just like that one. The grooved protrusion on the end is to tie the paper cartridge onto the bullet. I'm not saying that was always done, but that's what it was for. The paper was nitrated so it would completely burn up when fired. The Sharps rifle is a breech loader, and the breech drops down, the cartridge inserted, and when the block was closed it cut the pigtail off the cartridge, exposing the powder to the primer, which in my case was like a muzzle loader primer placed on a nipple. There are also Sharps rifles that had paper rolled primers that worked much like a toy cap pistol to advance the new cap into position, thus making the rifle or carbine even quicker to load. Not everyone loaded the same way, but the one on the left is tied like I'm talking about. On the left the breech is closed, on the right open.
When opened the block pulls straight down. Then when closing it cuts the paper pigtail off.