In his Shotgunner's Notebook the late Gene Hill had this to say of the 16-gauge:
"There's no really valid argument in the 20 vs. the 16 from a pure balistic standpoint. You can wave almosts around all you want, but it makes no difference to the quail or grouse whether he's centered by an ounce of 8's from a 16 or a 20.
"The 16 being a bit larger in bore diameter (.670) than the 20 (.615) and likely a shade heavier would certainly have less felt recoil, but not that much less, especially to the gunner swinging on a fast-moving blur of brown feathers. There really isn't much logic to the 16, but I find that there's still a lot of love.
"Where the 16 excels, and again this is purely in the eyes of the beholder, is in its almost perfect lines. Where the run-of-the-mill 12 tends to be bulkier and heavier than it has to be, and the ordinary 20 is as often too light as it is too coarse, the true 16 is a fine example of the creed, 'form follows function.'"
By "true 16" Gene meant a gun with a 16-gauge barrel on a 16-gauge frame, e.g., the Browning Sweet-Sixteen or Francotte, both manufactured in Belgium; the Ulm from Germany, or various products of Britain and France. Unfortunately back in the day many of the 16-gauge guns manufactured here in the U.S. were hybrids: 16-gauge barrels mounted to 12-gauge frames, and accordingly were heavier than a true 16. As a result, they were disadvantaged against the lighter 20-gauge whose manufacturers said that anything the 16 could do, the 20 could do just as well, and be more comfortable to carry.
However, as Gene went on to say, "When we insist on lightness and speed, we have to be fully conscious of the fact that we are likely to overswing the bird, then check a bit, and then either do it over or risk missing behind. The light, fast shotgun is a very hard taskmaster, but the momentum of the heavier gun tends to help us erase a little mistake here and there."
My Browning Sweet-Sixteen (manufactured in 1926) is a joy to carry and to shoot, and I believe that there is no finer shotgun (in gauge or make) for use on upland birds. To me it is the ideal balance of form v. function. So long as manufacturers continue to make ammuniton for me to feed it, and so long as God allows me, I'll be afield with it each Fall.