Sir Joseph Whitworth is quite the famous name in engineering circles, credited with the development of such things as Whitworth threading (the first standardized thread pattern) and engineer’s blue. When he decided to make a rifle, he decided that he could make flat surfaces more precisely than round ones, and chose to design a rifle with a hexagonal bore and mechanically fitted bullets.
Video: [15:19]
https://youtu.be/Hi-S_horZGk?si=
Ian makes a correction to the original video: [7:18]
https://youtu.be/cUd2RQGfL7E?si=
Spinning helps with range and accuracy. How destructive a bullet is more because of its weight and speed, which is controlled by how much propellant like gunpowder.