The tapered bullet base, like the stern of a ship, allows the projectile to maintain flight stability at extreme ranges beyond 500 yards for instances when the initial velocity, though supersonic, is insufficient to keep the bullet from going 'transonic'. There are those that theorize that the crossing of the sound barrier as it slows causes the bullet to lose stability. Many disagree with this premise. All agree, though, that if the muzzle velocity is sub-sonic the boat tail will improve flight stability. The hollow point of the jacket, combined with the boat tail, improve the ballistic coefficient of the bullet.
Air is a medium just like water.
The more streamlined the shape of a bullet the less air resistance will act to slow it down. The same principles apply to speed boats. Non-jacketed bullets are of various lead alloys, formed either through casting molten lead or cold extruding soft lead wire. The more antimony, and to a lesser degree tin, that is present in the alloy the more velocity the bullet will stand without badly leading the firearm's bore. Softer lead bullets pushed to excessive velocities will also be inherently inaccurate due to base deformation caused by the hot propellant gases pushing them through the bore.