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.