All I can say at the moment is that it is definitely not from the civil war era. That being said... it appears to be a .38-caliber/9mm breechloading rifle or pistol bullet. The presence of the shallow cavity in its base indicates (but is not 100% proof) that its basic form is from the last 30 years of the 1800s rather than the 20th-Century. However, it might be a modern-era imitation of an older bullet, made for shooters of antigue guns. It's going to take additional research, because I can't even find a match for it in my primary research material for post-civil-war bullets.
