Your stock is already "finished" so you are not trying to remove tool marks etc, just have to get rid of the current finish (which I don't remember being too deep).
I would start with 100/120 grit - won't load up as fast as a finer grit.
When I'm building a rifle that's about what I start with after I'm done with the files/scrapers.
Then clean up the lines/marks from the 100, with 220 - then 320 (which is pretty smooth, depending on the finish you are looking for).
When you stain, I then sand the "whiskers" (stain raises up little bits of wood which if not "knocked down" will leave a grainy look) between coats with 400-600 grit (again, depending on how smooth I'm looking for).
Most original rifles were not "sanded" (sandpaper was not readily available), they were often finished with scrapers (which can leave a pretty smooth finish, but certainly nowhere near what you get if you keep working up to grits above 1000).
If you are looking for more like an original "might look", I certainly wouldn't go above 320 grit - between coats of stain, when dry, you can rub down the stock with a rough material (denim works well) and it will smooth any whiskers.
You aren't thinking about going at your pretty new rifle with sandpaper just yet are you?
If you are, plan out all your changes/modifications first. As was suggested here, if you plan on adding a patchbox it's best to do that first and then refinish and if you are not familiar with inletting metal into wood, not something you want to try on your new gun before alot of practice - it's easy to make a mess that can't be easily fixed..