S&W makes a .50 caliber revolver- weighs something on the order of 5lbs empty- it's so large it only holds 5 rounds- I live in bear country- only take a Winchester Defender- 3" inch magnum loads- much more likely to hit what you're aiming at unless you're an accomplished marksman with the pistol- a charging bear is no time for practice!
The Ruger's hefty, but fairly small. Personally, I'd rather rely on a 45-70 Marlin with heavy bullets, or a 12 gage - but I don't think they'd do good in a backpack [Although one of my Marlins is a takedown]. And if they did, I'm not sure you'd get it together in time for Br'er Bear if he came a callin'.