i bought a nice CZ, 300Win magnum.
see, the reason why i sue a 300Winmag is because one thing leads to another, first deer, then bear, then moose or elk, then a grizzly one day. so a 300Win Mag will take out any animal, and i wont need to practice with 3 or 4 different calibers. It can even take out a bison. (you can hunt bison at a farm in Maine)
now, as far as black bear calibers, anything that works for deer (except an sks, although that might work too) should work for a maine black bear. (a .223 wont work) i only know one person that uses a .223 to hunt deer. he should not be hunting.
there are some big black bears, but most of them weigh in at about 200lbs, 250lbs would be a above average, but you can find them. most of the bears you might shoot would be 200lbs or under.
I know a lot of people that use a Swedish mauser, that bullet is perfect for deer and black bear. They are cheap rifles, extremely accurate (many competitive shooting people use that caliber and rifle), they are all in really good condition, and ammo is pretty easy to find, since they still make modern rifles that use that caliber 6.5x55
if you wnat to talk to someone that uses a 6.5x55, go to Plymouth bay outfitters on route 3A, the skinny dude, he told me he uses a 6.5x55mm