For work like this you need a collection of sanding blocks in various shapes, and sanding sponges, possibly cut into custom shapes.
For faster removal this is one of the few situations where small rotary tools like a Dremel will come in handy. Almost certainly you will find all sorts of shaping and sanding attachments suitable for crafting rifle stocks. I suspect a single web search for "dremel gun stock" will yield all sorts of opportunity for parting with your money.
Once you've shaped and sanded down to some level with a rotary tool, then I think you should switch to hand sanding using sponges, and store-bought and hand-made blocks. Again, don't reinvent the wheel here; people have been making weapon stocks out of wood for centuries. See what other people are using, possibly on another SE related to gun-making.
There is absolutely no tool better for gauging fairness and smoothness (all paramount for rifle stocks) than the human eye and hand.
@<username>. it's OK, we'll probably all find it... B) oof... that's complex! I've never sanded a gun stock before, but my guess is you'll have to do it by hand. You may still find card scrapers useful, but you may also end up with flats doing that - I'd wait until those with more experience chime in. C) That's a nice looking piece of work so far! Keep at it!