Real boiled linseed oil (actually heated to above 375° F) based varnishes have been used as a gun finish for hundreds of years and is similar to the varnishes used on fine violins including Stradivarious (it was even used by the Egyptians as a wood finish long before guns were invented) - it is not the same though as today's hardware store "boiled" linseed oil which is not heat boiled at all, but just has chemical (mostly manganese) dryers added - the problem with using manganese as a dryer is that the oil drys from the outside in and the surface skin that forms actually inhibits proper drying - period boiled linseed oil varnishes used lead as a dryer and that drys from the inside out thus no surface skin to impede the drying.
Boiled linseed oil varnish was the most common fire arms finish up through the mid-1800's when "greased" (most likely oiled) finishes became more popular. Some gunmakers also used a spirit based varnish - gun maker Eric Kettenburg has done a ton of research on both types of period finishes and has published his findings. Bill Knight aka the mad monk, has also published a tome on period finishes that is very well researched. No where in the period documentation have I read of 18th/19th century gunmakers using raw linseed oil for finishes - it is apparently an early/mid 20th Century innovation used for high production of military arms .
The common gunstock finish of the pre-1900 period was often two part - very thinned down seedlac/buttonlac (available from suppliers such as Kremer Pigmente) was used as a filler first and then the linseed oil finish was added - the combo while not waterproof was/is heavily water resistant.
FWIW I've used the same finish on my guns and knife/hawk handles for over 40 years and while it takes a bit more care than some modern finishes it works well ( depending on usage I re-apply a coat or two a couple of times a year). If you'd like to see what such a finish looks like check out the tomahawk handles on my site -
www.wrtcleather.com or look at the guns on Eric's site.
The linseed oil based varnish of the day was made by taking high quality cold pressed linseed oil and boiling it with the addition of various resins and white lead as a dryer. Then they often added air thickened real gum turpentine as a thinner (available from artist suppliers such as Dick Blick). When using this type finish it must be put on in VERY thin coats and allowed to dry between coats - lots of room temp or above moving air (a fan) and sunlight (UV) speeds the drying immensely. When done right it usually takes me only about 3-4 days to do a proper period finish, not the several months that is often touted for period oil finishes.
As for the reddish finishes - it was in fact often done by adding reddish tints such as dragon root to the finish - it is not just due to aged oil which usually goes blacker on wood rather than reddish - the so-called British/London Oil finish, used by gun makers such as H & H included/includes dragon root. On the other hand the real dark almost black stocks seen on some originals are usually due to:
1) the use of Aqua Fortis (ferric nitrate) as a "dye", mainly on maple stocks - if not properly neutralized it will continue to darken due to UV exposure
2) Darkening of the linseed oil finish due to UV/oxidation
3) The use of asphaltum or pine tar in the finish
While no one makes an exact copy of the period varnishes on a commercial basis these days that I know of, Tried and True's oil varnish is close and can be easily made PC by adding lead stephanate as a dryer (some art supplies offer it or you can also buy white lead from some suppliers) and adding a bit more resin/rosin which is also available from suppliers such as Kremer Pigmente - it takes a bit of heat to combine them, but an old crock pot/fryer at around 400° F will do that easily - just do it with PLENTY of ventilation or outside.....you can also get real cold pressed linseed aka flaxseed oil from suppliers including health food stores and start from scratch .
FWIW - The usage of raw linseed oil on 20th Century firearms has nothing to do with pre-1900 gun finishes......seems they were smarter back then
and tung oil is also a 20th Century finish, widely used by the Japanese on their firearms - BTW-tung oil can be a cause of allergic reactions for some folks....