The best for walnut is alkanet root oil, it will improve any grain and I have seen several times where the wood came out looking one grade higher than before.
This will give a slightly red hue to the wood which I like. I would sand back some as you are about to lose figure not increase it with the stain. Whatever finish you use first put on a couple coats of artist grade linseed oil, this pops the grain like it is whet and improves any finish. A dark filler will also enhance the grain. Herters French Red is good but on an open pored stock like yours it may need several applications and then becomes too red with out heavy sanding back. I just add burnt umber to a good silicone sealer like silisol. Polishing the wood with rotten stone will make the grain come out nicely, I do this during the finishing process so it is polishing the wood not just the finish. Last stock I did had so much chatoyance I wondered if it would scare the game away.
