I would suggest trying different stains and finish on scrap walnut first. The stock will darken up a lot when you put your finish on it. Walnut is already dark, and stain may just muddy the grain. I restocked a shotgun with a walnut stock and used Tru-oil. I test Birchwood Casey grain filler on a scrap, the used Tru-oil and it was very flat looking, no depth of grain. Then I just used several light coats of Tru-oil, steel wooling it back to bare wood between coats until the grain was filled. Then I put several coats of Tru-oil on and lightly scuffed with steel wool between coats. Finally I cut back the shine with Birchwood Casey sheen reducer. I didn't use any stain, the stock came out great. Practice with different products and techniques on scrap before applying to the stock.