Guys! Shhhhhh! Yer givin' away all of Vaughn Trueman's secrets! Seriously, that is THE way to go to get a 'new' gun to look like an 'old' gun.
Back in my 'Skinner days we would buy CVA Mt. Rifle kits in .50, use the same method described above, add a handful of Tea-soaked Brass Tacks on the stock and defy anyone to pick out an original from a kit gun when laid side by side. Most couldn't. My Navy Arms 12 SxS got the same treatment and I was offered mucho denaro many, many times for it just because of the 'old' finish, still have it and still laugh about it!
Yep, Birchwood-Casey's 'Plum Brown' is a real easy way to get the effect you want and it is so easy to use, but of course if you really want a gunsmith to do the work, Vaughn is one of the best and can probably make it look like it just came out of a Lineshack Cabin up on the High Lonesome!
Best regards and good sanding!
'Ol Gabe