A .308 is simply not large enough to stop any charging animal larger then a deer/black bear... even a charging 400pd plus black bear it would not put it down to the ground unless you hit it with a brain shot!
Trying to stop 1000 pounds of charging hungry bear with a 180 grain bullet is like trying to sink a battle ship with an ak.47!!
OPTIMALLY If I were you i'd get a .45-70 marlin with some good modern loads like buffalo bore which push out a 430 grain hard cast lead bullet at 2000fps and garret's 540 grain hard casts pushed out at 1550 FPS. Or an FMJ or Brash punch bullet would work too.
You could also get a .50 beowulf upper reciever?
http://www.impactguns.com/store/AL-U-BEOOM.html
and slap that baby on.
Look at the girth on that barrel!!!
The .50 beowulf is comparable in trajectory and power to a factory down-loaded .45-70 bullet or 12ga rifled slug. I'd keep the .50 beowulf upper in a nice silicon gun sock in a closet somewhere oiled up and ready to go. If you feel like yoru gonna be trekking through bear country or you sense something weird is going on and you think a bear is near slap Beowulf on there or if your gonna be hitting the hay i'd slap beowulf on there and leave it right next to me.
Other things I would consider for bear defense:
1. Buy a can of bear spray for 20bucks... good for deterring bears that aren't trying to kill you.. this way you wont have to kill every bear that approaches your cabin and you wont feel like a scumbag for killing every bear that comes by to sniff what you've been cookin...
2. Also you should mount a good solid LED water proof flashlight atleast 120 lumens so you can light up any man eaters lurking in the bushes... bears hunt/scavenge at night sometimes... and you don't wanna be fumbling around with a flashlight in one hand and gun in other while the bear is about to charge at you
3. Nice bright laser would be good too for getting the gun on target quick if you DON'T HAVE TIME TO CHEEK YOUR RIFLE
4. If you have a good alert puppy... doesn't have to be a big mean dog just even a small dog that has good smell and can alert you if something big and mean is coming... I have a 30 pound dachsund, and her nose is so sensitive and she's very territorial so when something is out of wack in her territory i.e. marauding animals or man eating bears or intruders.. she barks like crazy... its like a free alarm system for my house.
5. Shot placement... for any animal that can CHARGE ... bull moose, brown bear, cape buffalo, big elk during rut, bison... it's best to take the first shot at the shoulder itself so it breaks the shoulder bone and will slow the animal down incase it does decide to charge giving you enough time to go for a heart lung or brain shot.