It has to be either biological, chemical, or physical.
I'm guessing the last two. Here's why. If one looks closely, there is very minor pitting, then scratches, and splotches. I'm guessing there was minimal surface rust, a chemical was used to remove it, this chemical also removing some finish but not the rust, and physical abrasion occurred when someone tried to remove the rust when the chemical failed to work completely.
Same approach I use when my wife wants the house washed. I use an Outdoor Cleaner to chemically remove dirt, as attached to a hose, the water also physically removes some dirt, and the rest I scrub physically with a brush. Except I'm smart enough to use a brush that won't cause scratches in the vinyl siding.
How much did this revolver cost?