I've read through most of the responses and I think most people answered as to what they believe is the most common cartridge people would have in America.
For me when I think about what makes America or something American I think of the American West and our popular culture. This would be clear to us and those abroad. To represent that I think you would have to look at the repeating lever action rifle, single action revolver, and something like the Weatherby Mark V. So the cartridges I choose to represent that would be 30 30 Winchester, 45 Colt, and the 300 Weatherby Magnum.
The 30-30 shows the epitome of the lever at a time when smokeless had come into play. The 45 Colt made the jump from black powder to smokeless and still has a hold in it's place. Nothing says look at me quite like the 300 Weatherby did in it's day. Just my 2 cents.
