It's actually quite easy if the stock is strong enough (I'm assuming you're talking about a hollow stock). I mounted a swivel on my Rem 7600 rifle stock.
I removed the recoil pad to examine the inside, and I was happy to find a solid "rib" running along the bottom and top. Then I just had to drill a hole straight through the bottom rib. For the front swivel I used one of the little clamps that wraps around the barrel.
If the stock had been totally hollow, I would have reinforced the area on the inside with epoxy or fiberglass.
The Flying Coyote
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Nil Sine Numini
Nothing Without Providence
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Ephesians 2:8-9
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