Need you to clean it a bit better, to see if it has any body-grooves. Also, please put a digital caliper on it to tell us its exact diameter. (Put it in the caliper's jaws "lengthwise" meaning bullet's nose pointing toward the caliper's main bar.) Your find MIGHT be a "dish-base" Burnside Carbine bullet, but that type measures about .55-inch diameter, even thought it is a .52-caliber bullet.
This is the most consistent measurement. I also look for grooves but I can't see any. Of course, in this soil, they may have dissolved. What's unusual , though, is its length .
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.