For what it is worth, this is based on two rifles.
I finished the stock on my most recent project last week using a ferric nitrate solution. Ferric nitrate is the active ingredient in AF and the chemical can be purchase in crystalline form from chemical supply houses. This allowed me to avoid having concentrated nitric acid in the house. The ferric nitrate is pretty stable in crystalline form and you don't have to worry about leaks.
Here is the source that I used:
http://secure.sciencecompany.com/product1.aspx?SID=10&Product_ID=6384I made a solution of one part ferric nitrate and five parts water, by volume. Distilled water is probably best to keep unknown chemicals in the water from adulterating the stain. I used a 3 oz. Dixie cup as a measure and have almost finished with the pint that it made up in November 2005.
I applied the stain liberally with a throw away bristle brush and allowed it to dry a little before heating. The color before heating will be a sickly greenish-gray and will make you wonder if you thoroughly screwed things up. I used a heat gun with adjustable output to cook off the stain. Good ventilation is key.
The others are right that you need to be careful not to char the surface. The more applications you use, the deeper the stain will be. I was satisfied with three applications on both stocks. Burnishing the stained wood with 0000 steel wool will highlight the curl and lighten the color.
Remember to neutralize the acid. I use a saturated baking soda/water solution that I brush on until the bubbling stops. Little nooks and crannies seem to collect the stain and will bubble more than the surface.
I used Formby's Tung Oil Finish (high gloss) applied with a rag a little at a time and rubbed in well on both rifles. Probably eight or so applications. Rubbed out with 000 steel wool with more oil. Then Renaissance wax.
Hope this helps.
Larry Luck