The coating on your rifle is probably excess cosmoline (kinda like vasoline) soaked into the wood. After cleaning as much off as possible, what some guys do is either lay them out in the sun and wipe them down every half hour or so as they "sweat", or wrapping it in a towel and leaving it outside or in a hot car.
Mineral spirits are a petroleum distillate (one portion of what oil is broken down into). They're more volatile than gasoline but less so than propane/butane/LPG and naphtha. Basically paint thinner--a good solvent for getting rid of heavier oils and greases.
Here's a chart:
