On a Burnside bullet, the term "dish base" means a wide very shallow flat-bottomed cavity. That sounds like what you say is in your bullet's base. About the length: You said your bullet's length is .74-inch, and a dish-base Burnside bullet's length is .77-inch. You said the soil where your bullet was dug is corrosive, so that could explain the .03-inch difference. Lastly, your bullet could be a FIRED dish-base Burnside. Althought the bullet was .55-ich in diameter when made, firing it from a .52-caliber Burnside Carbine compresses its diameter down to .52-inch. Your bullet has the same "shape" (including the nose's curvature) as a dish-base Burnside bullet. Of course I could be incorrect about your bullet's ID, but now you know the reasons for my "best guess" is, it's a (civil war, yankee-made) dish-base Burnside bullet.