The frontier rifle is based on a jeff cooper design scout rifle, iI would prefer the 243 or 308win to the short mag. I myself plain to transfer my 357mag marlin lever into a scout setup.
Now on to bear, the polar bear is the species of bear that starts to hunt you during the hunt unless of course you run into some rogue other species.
My former Vo-deck teacher used a 300mag to take his black bear, I heard stories from my former college roommate out of Johnston PA about bears charging and not going down after a 7mm that it took a 338mag, I was also told a story were a bear absorbs 1 turkey load, 6-357mag hot loads and cased a friend to soil himself.
Now I consider my 270win to take a bear as long as i use large bullets and i would of course back myself up with my 44mag srh, or my Mosi M-44 7.62x54R with the long bannet would help, I have seen guys in trees shoot bears with arrows and even saw a crazy old add for a bear spear hunt. Spear hunt not my style expecially now that we have all these advanced rock-chuckers now a days.