I think you should be considering ONLY a partner who is armed with a 12 gauge PUMP Shotgun, loaded with slugs, if you need a back-up for bears. They do make folding stock models( that may not be legal in Canada) that are nice packages to carry on a sling over your back, and can be fired pistol style. However, they do weigh more than 8 lbs. and definitely fit in the category of " gear I left behind ". Far better to have a hunting partner who is armed.
Many of the guides are now substituting Marlin Model 95 Guide guns in .45-70, and .450 Marlin, with short barrels, that are ported at the muzzle, and take-down for easy transport, replacing their older Winchester Model 70s, in .375 Magnum.
With stout reloads and heavy bullets the old .45-70 makes a powerful short range firearm. One was used with 420 Corbon Bullets a couple of years ago to kill an African Buffalo, and the bullet passed completely through the chest and killed a second buffalo on the other side of the one that was shot, because neither the Gunwriter, or his PH was able to see the second buffalo. The .450 Marlin Cartridge was specifically designed for this kind of work, and the factory ammo is designed to give deep penetration, while expanding. Velocity is very close to the older .458 Magnum. As much as I love my .44 mags., they are not bear medicine for wounded Brown, and polar bears, even with a 300 grain bullet.
The technical improvements in bullet construction in the past 10 years has been awesome, and few people yet understand how much better the bullets are that are being produced for killing large game.
And, there is always the S&W .500 Magnum cartridge, that is also available in rifles. I have not heard any firsthand reports on it being used on game, but the large frontal area of the slugs, along with the velocity, has to be able to get the attention of even the largest bears.