where to start in modeling a rifle stock?
The rest of the rifle would be a good start ...
In terms of how best to convey your creativity into a model, I would suggest using a Surface. Functions like Lofted Surface and Boundary Surface give you good control in a way which intuitively corresponds to the model itself.
Personally, I would setup a number of planes which cut through the stock like a guillotine (normal to the rifle's barrel). Then, sketch curves representing the cross section of the stock at these 'slices', like the bulkheads in a boat or aircraft. Remember you can sketch half of the cross-section, then mirror the entire stock when finished.
Next, add sketch curves running the length of the stock which act as Guide Curves for the surface (like longitudinal stringers in a boat or aircraft).
It can be a trial and error process, so give yourself plenty of options with regard to these shaping curves.
Once you've got a surface you're happy with, look at using tools like Thicken to produce a solid stock.