Final answer:
The .410 gauge/calibre, which is a caliber measurement, has the smallest diameter bore among the options provided, which is .410 inches.
Step-by-step explanation:
In the context of shotgun gauges, the .410 gauge/calibre is actually a misnomer because it is a caliber rather than a gauge. It is the smallest in diameter compared to the other gauges listed, with a nominal bore diameter of .410 inches. When discussing shotgun sizes, the term "gauge" refers to the number of lead balls the size of the gun's bore that can be made from a pound of lead.
The larger the gauge number, the smaller the diameter of the bore. This means a 28 gauge has a smaller diameter than a 20 or 14 gauge but is larger than a .410 caliber. Thus, the correct answer is the .410 gauge/calibre, which is not actually a gauge measurement but rather a caliber that signifies the directly measured diameter of the bore.