A .45/70 "government" cartridge might be okay for that. A wider case than a 30 aught 6, and almost as long.

Look up the Marlin "guide gun." My friend jokes that he uses his "to hunt Chevys." And I believe Henry Rifles also makes a .45/70 now.

Or maybe use a .458? I vaguely remember reading somewhere that at a zoo in Ohio they keep a couple of .458 Win.Mag. rifles for their polar bear house.

The .338 Lapua shell is designed to resist Arctic weather for combat, but its bullet is too small, and WAY underpowered, for a CARNIVORE the size of a CAR.

The Canadian mounties gave up their old Lee Enfield .303s for polar bear maybe 2 or 3 years ago? They use a .308 Winchester now. And the Canadian Rangers only started replacing their old 303s this past summer (2018). Run a search on the "Canadian Ranger Rifle," both the Rangers and the Mounties use it for polar bear.

Personally, for something THAT big, that hunts YOU, and can come at you at TWENTY-FIVE MILES AN HOUR???

I want to AVOID THEM. And when I can't, get a CANNON. Or a bazooka.

Hard to swing one of those around in the doorway of your cabin, though.

Anything from a 30-30 up will do it.

303 is a .311 projectile. Or a 7.62mm.

That's the same as a 308 3006

Idk case length off hand, 53mm maybe I aint digging mine out, but they are a powerful round.

Don't kill polar bears man. They are rare beautiful creatures. We can make more people, just let your asss get eaten.