When it comes to a penertrating oil, get
some that does not have a lubricant in it,
like the stuff that comes from Justice Brothers,
i.e. auto part store ( I'm a mechanic in
Minnesota, and we know about rust and frozen
bolts, if that does not work, you are already
to replace the stock, (by doing it this way
you will ruin the old stock but not the gun)
take a dremal tool and grind away the wooden stock around the bolt shank where you could get a pair of vise grips on it, and be able to get
right at the threads with the penertrating oil.
the most extreme method would be, after the stock has been grounded away from the shank of the bolt, heat it with a heat gun (NO Torch)
then hit the shank with freon (R12) not R134A,
R134a is amonia base, and would be not good to breath in, by spraying R12 on the shank would shrink the bolt threads making it easy to come out.