I only have kill experiences on 3 deer using Powerbelts. It was about 10-12 years ago with a CVA OptimaPro. I was shooting 348 grain AeroTop Powerbelts and achieved 3 quick kills on one nice 8PT and 2 doe. All were complete pass throughs(lungs) with all 3 deer dropping within 10-20 yards after hit. On bear though I'd be more comfortable with a bonded lead or all copper bullet such as a Barnes.
I've been killing deer with the 300 grain Barnes Expander MZ in my Omega for a few years now. After less than ideal bloodtrails from several different bullets I've found a keeper. They are pure devestation on whitetails and can't imagine they wouldn't perform well on bears.
I’m a great believer in luck, and I find the harder I work the more I have of it.
Looks like Barnes would be a winner. I use 300 gr. Sierra .458 HP bullet seated in a Harvester .458 - 50 caliber Sabot and 100grs. 2F Triple 7 powder in my CVA Optima V2. This gives me about the same as my 45/70.