For a pretty easy (NO boiling required) historical type varnish finish, get a quart of the Tried and True "Varnish oil". Now, I have found that it is simply too soft still to be a surface varnish (not enough resins), so, resins can be added.
This is a very unscientific recipe, and you'll have to experiment to find what is just right. I just happened upon a pretty good formula right off. Pour out (or use up!) about a quarter or so of the can of Tried and True. In a glass jar, pour in some turpentine (about the same amount that you poured out of the can) and add rosin. Stir it up, and keep adding rosin until it won't dissolve any more. This will take several days or even a week or more. When the turpentine is "full", pour it into your can of Tried and True and mix it up good. Now you have a varnish that will be hard enough to use as a surface finish. If you get it too hard when dry (not likely) you can add some more oil. If it's still too soft, simply add some more rosin. Practice with it before you commit to finishing a gun with it. I've had shockingly good success with this as a finish. I've done modern pistol grips and other stuff with it too, and it holds up quite well.
For a purely in-the-wood oil finish, nothing on the surface, here's another relatively easy way to go. Get the Tried and True "Danish oil". This is a good quality pure linseed oil that is "pre polymerized" (apparently stand oil???) to a small degree. Apply this in THIN LIGHT coats. Allow it to sink in and make sure to wipe ALL excess oil off the surface. With each LIGHT application, put it in the sun to dry. Sunlight is absolutely crucial and necessary. Do this over, and over, and over, and over again to fill the grain (some people use sanding dust to help fill the grain. I don't, but you may find it helpful to hit a chunk of walnut wood on a sander and collect the dust to rub into the open pores of the stock along with the linseed oil). When it finally gets built up near the surface, put a coat on relatively heavy, leaving it to stand on the surface. Put it in the sun and let it nearly dry, then cut it off the surface with burlap, across the grain with a shoe shine motion. A small amount of turpentine on the burlap may help some, but don't use too much. THIS is the "hand rubbed" part of a hand rubbed linseed oil finish. It requires a LOT of elbow grease. When it's all finally off the surface, every nook and cranny, put it back out into the sun to fully dry, dry, dry. You will probably have to do this one or two more times to get the grain completely filled with a smooth surface.
As with any linseed oil based finish, the applications need to be LIGHT (none of this "flood and soak" stuff), and each coat needs to DRY in the SUN. Full sun for at least a day on all surfaces. I can tell when it's dry by the smell difference. When in doubt, leave it in the sun for another day or two... or three or four.
