Most Alaskan guides do not highly recommend slugs. The hard Brenneke slugs are acceptable, but no one would suggest standard slugs for large bear protection, not really a great choice for smaller bear. Shotguns are sometimes used because they are cheap and accessable.
If I were in big bear country I'd carry a short barreled CRF SS bolt rifle chambered in 30-06 loaded with heavy 200-220 gr Nosler Partitions. A 375 mag would be a better choice, but I'd still go with the 30-06 primarily because of recoil and the fact that I can get 5+1 mag capacity vs 3+1 with a magnum. I'd also use GOOD bear spray as my 1st line of defense
http://www.fs.fed.us/pnw/pubs/gtr152
A good read. A study conducted by the Alaska Game & Fish Dept on bear protection. My choices are heavily influenced by their findings.