People who cast their own lead bullets describe your 3-groove Minie bullet as a "short-pour" bullet. You bullet was cast in a mold whose filling-hole was at the tip of the bullet's nose... which produces what is called a nose-cast bullet. In your case, not quite enough molten lead was poured into the mold to fill it completely. (In other words, the amount was "short" of the amount needed to completely fill the mold.) The molten lead contracted slightly as it cooled, causing the indention with a little teat at its center.
Our fellow T-Net member Jdug found the three civil war .36 Savage pistol-bullets in the photo below, which were cast in the same bulletmold. (In this case, it was a base-pour mold, instead of a nose-pour mold like yours.) The one on the far right is a "short-pour" bullet.
The other photo shows two dug civil war Colt Revolver "short-pour" bullets which were found with the complete ones in the top row.
