Knowing from experience, it can take a lot of work to get a bullet down a foweled musket barrel especially with just the ram rod, I’m surprized you don’t see more muzzle loader projectiles with heavy nose damage.Definitely a Confederate Gardner Minie-bullet, which has been fired and thus is a bit deformed. What you are describing as a "groove around the top" is the imprint of the rifle's ramrod, caused by having to use a lot of force to ram the bullet down a gunbarrel heavily "fouled" with gunpowder-ash.