What type of ammo (38; 357), what bullet weight, and what distance are the sights on a Model 13 calibrated for Point of Aim to equal Point of Impact? Mine seems to like 125gr 38 spl at 20 yards. Does S&W use a "default" ammunition at a "default" distance when they create a fixed sight revolver?

Not exactly. "Calibrated" is not a word I would use with S&W fixed sights. However, I think that they do have some idea of what they would like to achieve. They just usually don't care that much whether they achieve it. I think most folks (myself included) believe that their intention is to have 158gr bullets' POI = POA (center hold) at 25 yards. With K frames, they often achieve it, but certainly not always. Also, even if you have no oddities in your shooting, firmness of hold can definitely affect zero in centerfire calibers, maybe even in rimfire.

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Formerly Model520Fan