If I had to name one cartridge as being the most American I'd start with considering which firearm is the most iconically American. First I considered the flintlock musket with which we won our independence but then it occurred to me that countless battles and wars have been fought with the flintlock musket by armies across the world so there is nothing distinctly American about it. Next I considered the flintlock long rifle which is most definitely American, however they played a relatively small role in American history in the hands of hunters who could afford them and some small specialized units in our early wars.
The M1 Garand was considered and there's no arguing that the M1 is a legendary American rifle, however it is legend among the gun community in the US and the rest of the world would hardly recognize it. The Colt Model 1911 is another strongly American firearm and I categorize it similarly to the M1 Garand.
I finally settled one firearm that would be recognized the world over by both gun folks and non gun folks as being distinctly "American." The Colt Single Action Army. Everyone across the world recognizes this firearm as being distinctly American as its featured in countless movies that span decades. It is a gun that played no significant role anywhere else in the world except during a fleeting but highly colorful period of American history that captured the world's attention. That pistol was chambered in 45 Colt so that would be my choice.
The M1 Garand was considered and there's no arguing that the M1 is a legendary American rifle, however it is legend among the gun community in the US and the rest of the world would hardly recognize it. The Colt Model 1911 is another strongly American firearm and I categorize it similarly to the M1 Garand.
I finally settled one firearm that would be recognized the world over by both gun folks and non gun folks as being distinctly "American." The Colt Single Action Army. Everyone across the world recognizes this firearm as being distinctly American as its featured in countless movies that span decades. It is a gun that played no significant role anywhere else in the world except during a fleeting but highly colorful period of American history that captured the world's attention. That pistol was chambered in 45 Colt so that would be my choice.