Well if you are wanting to duplicate the original finish of most Amercian made long rifles you'd have to chew tobacco. Many Gunsmiths found that Tobacco juice spit onto the wood and rubbed into it brought out the grain.
Some Master gunsmiths used a Mercury based chemical that vitrified the surface of the wood and gave a silvery sheen to maple, the result looking like a Silver Tabby's fur. The flame grain graduated from an odd almost black through many shades of silvery gray.
I think the chemical was called Mercury Bicarbonate, I learned of it first when reading Edgar Allen Poe's "The Gold Bug", the process had been used on the pirates treasure chest they find in the story.
The grain was brought out and made more prominent by heating the wood over a flame.
A method used to make less nicely grained wood take on a flame grained maple appearance is to soak a cotton cord in pitch and wrap it around the stock then set it on fire. I'd practice alot before trying that one. It does look very nice if done properly.
A hand rubbed oil finish over these staining methods would usually be Boiled Linseed oil applied many times and hand buffed daily till it sets properly. That generally took too long for mass produced guns, thus the French varnish method.
A method you are unlikely to see these days was the application of a paper thin layer peeled from a sea turtles shell. It has a plastic like appearance with a reddish mottled look. Its transparent enough to see the grain under it if well preserved. This Turtle shell coating sealed the wood against moisture very well, but few except nobility could afford it.
PS
"Aqua Fortis" is Nitric acid solution. Seems I'd heard of its use before but had forgotten it till mentioned.
Probably a lot less dangerous than the Mercury Bicarbonate.
I use a Walnut stain of my own manufacture.
I collect walnuts from my Black Walnut trees. I leave them sealed in plastic bags till the outer husk rots.
I then put the husks in a strainer and pour scalding water over them, catching the stained water in a bucket. I pour the stained water in flat pans and let the sun dry it until it forms an oily crust on the pans then wash the pans out with 70% rubbing alcohol.
The inside of the plastic bags often contains a lot of the oily residue and this can also be scrapped out and added to the mix.
If you boil the water away the stain will have a more reddish brown look.
I sometimes just save the rags I wipe the pans out with and when needed just pour alcohol on them and rub down the wood with them. That works okay till the rags begin to rot.
The alcohol stain solution can be allowed to evaporate for awhile to form a concentrate.
The thin solution is good for evening out the color of a stock that has sap wood or is uneven in shading for other reasons. Using many applications of the thin solution allows you to make the stock as light or dark as you please.