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=
Polygonal rifling is commonly found on nearly all Glock pistols along with many of Heckler & Koch’s guns just to name two brands. I think technically the mechanics of the Whitworth rifling differentiate it from modern polygonal rifling, but that’s me being very technical and pedantic.