It appears to be an unfired civil war era .44 Colt Revolver bullet, cast in a bulletmold manufactured by Colt's company. You didn't give us any size-measurement... but if it is a .44-caliber bullet, it should measure about .46-inch. The next smaller size of Colt bullet would be 36-caliber, measuring about .38-inch.
You asked about the "third ring" on your .44 Colt Revolver bullet's nose. No Colt bulletmolds have a ring (or actually, a groove) at that location... so, iIt MUST have been made by somebody with a knife, just fooling around. It serves no purpose.