Here's what I'd do.
Stock: after the wood has been shaped, carved, sanded, etc, etc, raise the grain and scrape it as the last thing before applying the stain, etc. Next, stain it with AquaFortis. Finally, since 18th century varnishes are not generally available, finish it with some modern varnish or an oil finish like the Chambers stuff.
Steel parts: file them down with a really smooth file (or sand to 220) and tarnish them with Birchwood-Casey Super Blue, which is rubbed off with 0 plastic wool and oil. This will leave a silver-gray finish. Super Blue works well on brass too, just wool it off and it will leave the low spots dark.
I've seen references to "russeting" smoothbore barrels well before 1780. The problem with browning American rifle barrels before then is that there is a Geddes gunshop (in Williamsburg) ad of 1751 that says they "blue" and "rifle" barrels. This implies that rifle barrels were blued, and flintriflesmith and the guys at CW say they have seen 18th century and early 19th century rifles with evidence of "heat" bluing on the bottom, where it didn't get rubbed off. The ad says nothing about "browning" or "russeting".
Hope this helps,
Andy
PS: I'm trying to learn to do it the "old" way, check out the link to the Photobucket album of the Martin's Station Gun Shop below.