Southeast Alaska Brown Bear rifle caliber:
Ruger American .30/06, 220-grain bullets.
In general, Southeast Alaska has smaller brown bear than the massive 10-footers further north, but there are some really nice, quality bears on the mainland in general and the Admiralty Island area specifically. The ability to spot and stalk and get pretty close allows for heavier grain bullets. Long shots with heavy bullets make drop estimations critical. As big as you can but still shoot confident and straight was the advice I was given.
If brown bear hunting was something I really wanted to get into, then I would buy a larger caliber rifle. In most cases, guides will require a large caliber for clients.