Some years ago Fine Woodworking magazine did a test on the moisture repelling capabilities of different finishes. I don't have the results at hand but I remember that linseed oil did little to keep out moisture from wood finished with it,but,as I said,I don't have the particulars of the tests with me. I finish most of my walnut furniture I make with linseed oil and or varnish and I like it because it is easy to repair and looks good.Use the best quality you can find to avoid any chance of yellowing with exposure to light,probably not too much of a problem with most firearms,but furniture is often exposed for hours to sunlight through windows.