Which shotgun has the smallest diameter bore?

The shotgun with the smallest diameter bore among the options presented is the .410 gauge/caliber. In the context of shotgun gauges, the lower the gauge number, the larger the bore diameter. The .410 is a unique designation in that it is measured in inches rather than being classified as a gauge like 20 or 28, which refers to the number of lead balls that can fit into a barrel of one inch in diameter. Because .410 is smaller than both the 20 and 28 gauge, which have larger bores, it stands out as the option with the smallest diameter.

The other gauges listed, including 28, 20, and 14, all have larger bore diameters relative to the .410. The 14 gauge is particularly less common, but it typically would also have a larger bore than the .410. Therefore, the .410 caliber is distinct in being the smallest diameter option available, making it the correct answer.