Nate,
All my guns are stained with vinegaroon, aka vinegar/iron stain. The best results I have gotten are from using cider vinegar and mild carbon steel or pure iron. I do NOT recommend using steel wool. Steel wool is mad from recycled automobiles and is by no means pure. there are a number of alloying elements in alloy steel that will frustrate your efforts to get a nice reddish-brown. Manganese adds a black color, nickel adds a blue-green color, as does chromium and copper. and all of these impurities are in the "steel" of steel wool. Try to get old iron nails or old rusty barbed wire. (There is plenty of that around my place in Benton!) fill up a large crock with cider vinegar drop in a fist-full of nails put a plastic bag over the top with some string to keep it tight and go do something else for a couple of months. Vinegar is a weak acid and it does not work as fast as nitric acid. It will take time and don't expect to see a lot of bubbling or fuming because it won't. My crock of vinegaroon has been on the shelf for over 20 years. I just add vinegar every so often so it stays liquid.
After letting it do its thing for a couple of months you will have a solution of iron acetate with some rust and iron nails in the bottom of the crock. Pour off a little of the clear solution. That is your stain. When you apply the stain the wood will look wet but the color comes in as the wood dries. No heating necessary, but usually I use several coats because the stain does not have as many iron ions per teaspoon of liquid as nitric acid stains do. I get colors that are very much like what Penn Dutchman has shown. If you rub the wood down with burlap the contrast between the dark and light stripes will increase. The color will become more intense when you add your finish.
You can nudge the color of vinegar/iron stain some. If you want more red follow the stain with some hydrogen peroxide (not period-correct!) This will shift the iron oxide more to ferric oxide (Fe2O3 reddish brown) rather than ferrosoferric oxide (Fe3O4 black). The faster you dry the wood the more red it will become. Leaving the stock without finish a long time will tend to blacken the stain. The red and black iron oxides are in equilibrium in the wood. Usually the result is a nice warm slightly red-o4range brown.
Every old farm had apple trees and used vinegar to pickle meat, eggs, vegetables, etc. I just don't believe that colonial gunsmiths on the frontier would spend good hard money to buy nitric acid that was shipped across the Atlantic from England, at considerable expense, when vinegaroon was essentially free and had been in use as a stain since the middle ages.
I have been using it exclusively for over 20 years. I like it.
Best Regards,
JMC
John Cholin