First off: There are no grizzley bears in Maine, so you don't have to worry about that.

Secondly, a 30-06 loaded with the CORRECT BULLET will kill any grizzley bear 100% of the time, under any circumstance. Those who say otherwise, simply don't know what they are talking about. A mammoth, 1000 lb grizzley charging you at 10 ft that takes a 180 gr. nosler partition from a 30-06 in the brain will die immediately. If you hit him in the neck/spine, he will die immediately. If you hit him in the shoulder, it will knock him for a circle, upon which you fire again. A shot to the body will change his mind very quickly about attacking you, upon which you fire again. The secret is to fire until he dies.

I know there are those who believe that you need a 375 H&H, etc. to get the job done, but that isn't true. A 30-06 has 3000 lbs of energy at the muzzle, and if that won't get you out of the situation that you are in, a bigger gun won't do you any better. Once the bullet exits (likely to happen with a more powerful gun), all that extra energy does you no good. Stick to a 30-06. Recoil is much more manageable, and if you need a second shot, time is much more precious than energy.

Any of the following callibers with the right bullet will serve you well:
30-06
7 Mag
35 Whelen
358 Win
45-70 govt

Technique and shot placement are most important. If possible, drop to one knee because movement or changing distances won't matter if you are shooting strait out into the bear. Aim for the head. If you go high, you hit the spine. If go low, you'll hit the heart, but keep firing.