this idea comes up continuously in various forums, the main problem would be safety/playability, the benefits of the spherical shape of a bb are that it produces a good amount of drag, and when it hits something it distributes the force over a large area (no puncture wounds in airsoft) moving to a heavier rifled round would make the impact area much smaller contributing to an increased injury rate, as well as raise inertia to inane rates (in order to see a significant increase in range and accuracy one would need to use a much heavier round at roughly the same velocities, hence raising kinetic energy)