this gun hadn't been shot in nearly 20 years
I took it out, put some rounds through it, and after getting home, opened the padded case to find this....






Can ANYTHING be done about this?
This is very sickening.  And I have no idea how this could've happen.

The root of the problem is a poor choice of stock blank by the manufacturer.
It was chosen for it's pretty burl pattern with little regard for the grain pattern at the wrist, (the thinnest point of the stock and the most liable to break.)
If you look closely, you'll see how the grain fibers run almost across and up and down through the broken part.

Yours almost looks like broken particle board!

It could be repaired with brass pins and epoxy in conjunction with a purpose-built clamping jig, but I think I'd replace the stock instead.
It's a little odd that it broke in the case and not while you were shooting it, though.

Thanks.  I'm already looking at new stocks for it.  Too bad it can't be fixed.  

Someone else who I spoke too basically said the same thing you did: Beautiful stock, but poor choice of wood with the poor grain direction.

The person I spoke to also said that there could've been a hairline crack in it, and that these last shots just did it in.  

And yes, it is odd that it happened somehow on the way home.  I've been shooting and handling firearms all my life, and do not abuse any of my weapons.
It performed flawlessly.  My first 3 shots cut a 3/8" group at 100 yds.  I only ended putting 10 or so down range, packed it up in the soft padded case, and sat (not dropped, or threw) it in the back floorboards of my vehicle..  There was nothing I remember from the period I pulled it out to when I put it away that led me to think there was remotely anything wrong with it.  
I get home, unload my vehicle, take the rifle out to clean it...And pull out just the butt end of the stock.
I believe a weaker man would've puked.


Someone else on another forum also mentioned that this split, in this area of the stock, is common on those particular Colt Sauer magnum rifles.
A damn shame I say.  

Any suggestions for a good stock, for such a not-so-common rifle, anyone?

What probably happened is that the recoil from those few shots jammed the receiver tang good and tight into the inletting, and then the heat from the chamber and barrel eventually sank back into that metal, expanding the tang and bottom metal just enough to pop the wood apart. Who knows?
Try Boyd's for a replacement; otherwise you might need to talk to a custom stockmaker about starting from scratch. Nice barrelled action to start with if you're going to go that route, at least. And you already know it's accuracy potential, so that's worth a lot right there.

Why don't you take it to an old cabinetmaker and see what he can do with a few pencil-sized brass pins and some epoxy first, though? If he's good, you might be pleasantly surprised.

Edited to add:
Nevermind. I'm an old cabinetmaker, (and good!).
And after taking another long hard look at your pictures, I'm pretty sure I would refuse to even try to fix it. It would be a waste of your money, especially considering it's Magnum chambering. Sorry.