20g
Shoot your friend thats a good one.
I would get, like others have said, the best slugs you can for your 20g. A 20g is better than hand to hand combat with a bear. You said you didnt want to invest money in other gun or pepper spray, i understand that, however most bear pepper spray can be bought for the price of high quality slugs. I think i would take both. When i'm in bear country and out for a hike i always make noise sing whistle beat on stuff with a stick, that way i dont spooke a bear. Might be worthless but it keeps my happy making noise. guess its the kid in me.