Modern "boiled" linseed oil, containing chemical drying agents, can be a fine finish for sporting arms where a shiny surface coat is desired - but that's not what was wanted on a battle rifle. Raw linseed penetrates the wood and eventually dries beneath the surface, giving plenty of time to wipe off any excess that doesn't sink in after a day or two. Result - a deep, durable treatment that enhances the wood without producing a shiny surface.
For another approach, here's some advice from James V. Howe's classic work -
Don't try this indoors! :o
