I just went and picked up a bunch of walnuts at my neighbor's house the other day.
Some claim to be able to dye cloth with it... I have never been that successful getting anything fast, though I have sure tried.
It is, however, FANTASTIC for leather. Imparting a warm brown tone, and it actually retans the leather (it is possible to tan leather outright with walnut hulls, though almost no one seems to do it). I find it greatly improves the temper of some of the cheaper "vegetable tanned leather", though it won't work miracles. Cheap stretchy leather will remain cheap stretchy leather.
I can't imagine any good use for it in gun work, really. Aqua Fortis makes a far superior stain, for sure.