Modern rifles use small calibre bullets.
If they were spherical, their low mass per unit cross section would mean that air resistance slowed them down excessively. This would shorten the range of the weapon and reduce its hitting power. Therefore, for small calibre bullets to meet military requirements for range and flatness of trajectory, their mass had to be increased and a more streamlined form adopted. This meant that the bullet had to be made much longer and with a tapered nose. Such a bullet will be inherently unstable if projected from a smooth bore.