For the 1st half of the 20th century, the roughshooter's favorite "little gun" was the 16 gauge. With an ounce of shot and 2 1/2 drams of powder, it was a perfect choice for upland birds. Double guns chambered for it were about a pound lighter than the equivalent 12 gauge, their handling was more dynamic, and their slimmer barrels gave them attractive lines.
From 1900 to 1950, the 12 gauge was a waterfowler's piece, and the 20 was for kids. This left the 16 as THE choice of the experienced upland bird shooter. In those days, it was called the "sweet 16".
Most gunners today are unaware of the grace of a gun with a bore measuring 2/3 of an inch, and have never experienced the 16's charms. Instead, they make do with a light 12 (which is good enough), or endure the vicious kick of an overloaded 20.
If you find a 16 afield, it's likely to be in the hands of a man with silver hair. He carries it because it's efficient, of course - but also because it whispers to him of different days and simpler times. When it comes alive in his hands he's young again, and life is good.