This was shot in one take so you are just gonna have to put up with my nuances.
As the title says, I believe I have stumbled upon how rifling in gun barrels came to be. Longitudinal, spiraled forging scars that were never removed from the bore of the barrel may have been an accidental discovery of adding spin stabilization to a projectile.
To see these particular barrels being forged, go to my video list and watch forged gun barrel method 2 (skelp method) and forged gun barrel method 3 (spiral wrap) and to see them destructively tested, watch test barrel proof testing.