My favorite configuration...16 "O" frames..

[ArtS]

I'll chime in since I have bought quite a few lately:

Parker 16 g DH 28" Damascus
Sauer 16 ga Hammer
Sauer 16g x 20 g rifle cape gun
Unknown German 16 g Hammer
German 16 g O/u combo gun
German 16 g Drilling
Sterlingworth 16 g (x2)
Ithaca 16 g Flues Gr3
Ithaca 16 g Gr 4
Ithaca Field 16 NID
Manufrance Ideal 16 g (Spectacle action)
Manufrance Simplex 16 g
Manufrance Robust 16 ga
H&H boxlock 16 g

And being shipped:

Verney-Carron 16 g
Browning Citori 16 ga

Truly my favorite gauge over many years. After shooting a well made 16, any 12 feels ungainly due to size unless heavy shooting. The secret is weight and balance. Modern 16's are generally just twelves with small barrels. All my 16's weigh 6-1/4 or less, except the combo guns etc. Many weigh in the 5's. I don't buy them if they are heavy. In truth, many 20's and 28's weigh more. I seldom shoot them with over 7/8oz and have seen no need to. I find the effectiveness of these as good as the 1 oz loads.

Edit
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Forgot the late 1800's Darne, the mid century Charlin and a 1904 Manufacture Liege that is a like new 6# hammer gun.