I would seriously consider just stripping the stock of its oil/dirt/sweat/blood whatever and staining with a Spirt based stain and topping it off with BLOicon or RLO. I know you'll get much better results then trying to stain over the existing finish. The end product will retain the original look but be stained, guys here have done it to their rifles countless times. The problem you will find trying to take the "easy" route will be anything you use wouldn't penetrate and will rub off in some spots and remain in others leaving a blotchy finish.
May I suggest "Chestnut ridge military stain" for the red/brown color you seek.
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