No secrets, Mike- just information that I got here on this great site.
I started with about half a small jar of plain old white kitchen vinegar to which I added a few old antique style iron nails. After a couple of weeks it turned sort of yellowish. I then filled it up with fresh vinegar and added a couple of rusty old nuts and some iron filings from my gun. After another week it turned brown. I filtered it to get rid of the junk at the bottom, then just plastered it on the stock with a folded piece of cloth. Before it could dry naturally, I used a heat gun on high to dry it. Looked like the color of a kid's cup of chocolate with milk in it. I rubbed it down with fine steel wool, then did it again. Gave a very nice dark brown color, but one without much depth, so I added just a touch of warmth in color and a great deal of depth with a wash of Minwax Cherry finish, which turned it all 3-D. As I mentioned, I'm now into the first coat of a multi-layered thin oil varnish finish.
All in all, I got pretty much what I wanted- a fairly dense, very dark look that to my eye looks era-correct. I know modern finishes are much more transparent and more attractive and bring out the curl better, but that isn't how the old rifles look to me- even those that haven't had the linseed oil oxidize black.
Clay
San Jose