Different salts give different types and grains of rust. Straight salt gives a red rust and grains are somewhat coarse. But it could have been commonly used by any backwoodsman needing to darken his gun in the field. All results with the cold rust methods depend a great deal on the humidity, how long you leave it on, how aggressively you card it off, and how many applications you use.
Backwoods alternatives: urine works as well as salt and you get to use the handy applicator too. Vinegar will darken bright steel, as will most fruit that is acid, from green apples to tomatoes. Just slice and rub. Mustard works well to darken the metal and stays put while it works. Then you've got cow manure soaked in cow urine- also makes a nice plaster. A little more aggressive but not as much as chicken droppings. I draw the line there. :shake: