It is one of several slight variations of civil war era US .52 Sharps "New-Model" bullets. The variations are distinguished by differences in the depth of the flat-bottomed grooves between the raised rings, and the version of base (flat solid, flat with nipped-off casting sprue, small cone, medium cone, large cone, small "plug," small teat, and large teat). Looks like this one is the "plug" cavity version. See the Thomas-&-Thomas "Handbook Of Civil War Bullets & Cartridges" bullet #114.