1918-2018 – 100 years of the Mauser 13 mm anti-tank and anti-aircraft rifle

[Vlim]

It's actually both. Winchester worked on a .50 based on the French 11mm and an upscaled 30-06. When they had access to Mauser tank rifles they noted the superior velocity and penetration and then tweaked their design to match the 13mm Mauser tank round, thus creating the .50 cal.