Looks like it was "pulled" or wormed due to the ring aroud the top, also you might try cleaning the flatened top a bit better, you might find a small hole. Soldiers had a small screw like devise that fit on the ram rod to "pull" loaded bullets. Often when they ended picket or guard duty, but sometimes during a rain when the powder got wet and didn't discharge during a battle. It could be that the soft lead fragmented when he cranked down on the "worm" it went too far in and damaged the end. Either way you just might be on a picket post or bivoac site, either of which can yield up more relics than a large 'hunted out' camp. Good luck