Well my nomination would be the .30 WCF or/and the .45 ACP. As far as the .277" bore being unknown to the rest of the world, I think I remember Winchester swiped it from China. And the .30'03/1903 Springfield was brought about because the 7x57 in Mauser rifles that the Spanish used gave us a problem in Cuba against our .30-40 Krag. Fun reading, Carry on.

Last edited by Joe; 02/19/20.
Shew me thy ways, O LORD: teach me thy paths.
"there are few better cartridges on Earth than the 7 x 57mm Mauser"
"the .30 Springfield is light, accurate, penetrating, and has surprising stopping power"