You will have to prepared to take selective shots with a 30/30 on a 500+ pound bear dont kid yourself. Tuck a bullet behind the shoulder and it can do the job of course. A frontal shot at 100 yards is a bad idea with a 30-30. I would not want to pass on an opportunity because I chose a weapon that was marginal for the task. A 250 or 300 lbs bear is ok but 500 or 600 lbs bear are what most dream of, and a quick kill is asking alot from the 30-30. In case you think attacks are not a possibility it is not so far fetched.

http://sports.espn.go.com/outdoors/hunting/news/story?id=2241024

It was only a 350 lbs bear that attacked the hunter.