Or you can buy a small quantity of ferric nitrate crystals and dissolve in water. Dampen the wood with this solution, heat it to get the colors to turn to red/brown, neutralize and finish.
Also you can use white vinegar to dissolve iron (no alloys please) like old barb wire or old nails. This takes awhile (like a couple months) until no more iron will dissolve and the whole concoction looks like an orange gunky mess. Then use it same as aqua fortis with iron dissolved in it. Again not for walnut unless you want a black stock. This is the sort of stain procedure that the muzzle loading boys use on Curly Sugar Maple and Red Maple to bring out the curl. If you get other metals like chromium dissolved along with the iron the colors can be perfectly hideous (often green) so test your solution on a piece of scrap from the stock first. (Give it several months to stabilize before using it on your gunstock.)
Better solution is to use alcohol based stains which can be mixed to get whatever color you like and it is easier to overcoat and blend to your taste.
