Thanks for posting. I have made red violin varnish from the traditional early 18th century recipe. It is a spirit varnish with lots of variations and not nearly as time consuming to make. It would have been used over a AQ stain for the best color. It would be too red otherwise. I think some old longrifles were definitely finished with some variation of this spirit varnish as you can see where it has chipped or worn off. I think this is most appropriate for the Lehighs.
As to the varnish in the article, that is a much more specialized varnish specifically designed for musical instruments. I wouldn't, and I don't think the old gunsmiths would have gone to that much trouble just to put a little finish on their rifles. Linseed oil was commonly available, and think that is what most of them put on their rifles, perhaps with a little resin and dryer(white lead) added. The recipe in the article seems very hazardous to make. I wouldn't bother with it given that it is intended to do things I don't need.