That bullet has the characteristic form of a casting error which bullet collectors call a "short-pour" bullet. When your bullet was being cast in a bulletmold, not quite enough lead was poured into the mold to fill it completely. That's where the term "short-pour" comes from. The air inside the incompletely-filled mold allows the lead there to form the characteristic small "teat" on the bullet below the mold's filler-hole. Your short-pour bullet was made in a "nose-pour" mold... which means, the molten lead entered the mold at the top (or "nose") of the bullet.
Another version of bulletmold is called a "base-pour" mold because the lead enters the mold at the bullet's base. Bullets from those varieties of bulletmold are (respectively) called nose-cast and base-cast bullets. The photo below shows two "base-cast" short-pour Colt .36 pistol bullets which a digger found among several properly-cast ones in a civil war campsite where a soldier was casting his own bullets for his revolver. Note the "teat" on the bottom of those two incompletely-formed bullets, and compare what's on the incompletely-formed nose of your bullet.