I cannot speak with any certainty on original Krag stocks, but most military stocks in the 20th century were sanded to somewhere between 150-220 grit "as issued", but smoothed out pretty quickly as they were handled/used.

Stocks were not stained, but finished with raw linseed oil.  The fatty acids in the raw oil oxidize and darken to a rich reddish brown color over time - it can take years.  You can give the oxidation a jump start by putting the oiled stock in the hot sun.

Best source for raw linseed oil I am aware of is health food stores.  They sell it as "cold pressed flaxseed oil".

Pure tung oil produces a similar finish.

Raw linseed oil and pure tung oil cure very slowly.  It can take a few weeks for the finish to fully cure.

"Boiled Linseed Oil" has most of the fatty acids removed, and has chemical dryers added to speed up cure time.  It produces a decent finish, but does not have the rich colors that raw linseed or pure tung oil produce IMO.

To apply it, working in a small area, put a few drops on the stock and rub it in with you bare hand/fingers.  Rub it in until it gets hot, then move on to the next area.

If you have an attractive grain pattern and you do not want the stock to get any darker, you can "freeze" the color by using MinWax Tung Oil Finish, which contains both tung oil and a varnish.  One or two coats of that over the oil finish will seal the wood and prevent the oil finish from oxidizing further.

Attached a few photos of a Czech Vz 52 rifle with a walnut stock that I finished with raw linseed oil

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