Hi Horserod,
Let me dispell a myth: linseed oil alone is a poor finish and a poor preservative. It has little water resistance, is soft, and darkens with age. Linseed oil is pressed from flax seeds and unless artist grade, it contains a lot of organic debris, which is why it darkens with age. You can wash the manure you buy in hardware stores to produce a purer product but it is really not worth it. It was used in the past because it was cheap and available, and with the exception of military guns, it was usually mixed with resins and driers to make a varnish. Is your gun an antique? If so, be careful that you do not compromise its value by doing any refinishing. Show it to a knowledgeable collector before doing anything to the finish. If it is just an older modern-made piece then consider touching it up. Does it have a hard glossy or semi-glossy finish, or a low sheen that does not seem to be built up on the surface. If the former, chances are that it is a varnish of some sort. The latter would suggest an oil finish. If a varnish, you might try something like Formby's Tung Oil Finsh, which is a wiping varnish that contains highly altered derivitives of tung oil (but no actual tung oil). It is readily available and very easy to use but remove the buttplate and try a small amount on the wood under the plate to make sure it is compatible with the old finish. If the gun has an oil finish then try something like Minwax Antique oil finish (which is an oil-varnish mix, I think linseed oil and varnish) that should provide better protection and look similar to the original oil finish. There are many other products that likely would work but the two I mentioned are readily available. I believe there is evidence that some old guns were finished with shellac. That is easy to test for by placing a few drops of denatured alcohol on the finish (where it won't show) to see if it becomes tacky. If it does it is shellac or some other alcohol-based finish, which can be touched up with another coat of shellac. I suspect that would be very rare, however.
dave