I've never done it to a hunting rifle but, did lengthen the butt stock on one of my AK-47 clones because all their butt stocks are for short statured people. I added 1 1/4" of wood gluing and doweling the wood for strength. I then shaped the addition to the contours of the stock and re-stained and sealed the wood. The addition felt so much better when I shouldered the AK. Of course, you'll have to determine what size the new spacer needs to be for your rifle.
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