If you're serious about a bear defense gun you should put a lot more thought into it than your initial post suggests. I'm not trying to be a jerk but as someone who has had an all-too close encounter with a big brown, I believe it is a serious subject and consideration and advice should be in accordance with that.
Let's start with the fact that in a bear defense scenario you are already waaaaaay behind the power curve. A bear determined to harm you will likely give little warning, especially in the bush/woods. So if you aren't going to have it in your hands then you have even more trouble. Therefore, choose a gun you're willing to keep in your hands as much as possible. To me that says relatively light and easy handling. For a lot of us, the short-bbl'd Marlin springs readily to mind but for sure and certain a lot of guys have no issue toting a synthetic stocked 24"-bbl'd bolt gun. Like so many other gun-scenarios, the key here is, what fits you, what feels good to you, what will you shoot well and hand-carry often?
When we say bear defense, what kind of bears are we talking? In my mind, there's a big difference between say a coastal brown and high mountain black bear. While grizzlies are pushing into the northern mountains of my state they are still very rare and it's mostly black bear running in the 150-250lb range. I'm not discounting the potency of a black bear but they're a significantly smaller critter than an 800-1200lb coastal brown or a 700lb interior grizzly. I've never felt under gunned in my mountains with a 30-30 but if I were back in Alaska I surely would.
If you give me my druthers of a gun to carry here, it's a 336 30-30 with factory ammo and/or a .357 Blackhawk. When I go into the woods the sling goes into my pocket/pack and I feel perfectly comfortable. I shoot the pistol-gripped 30-30's very well and I love the way they carry and handle. I much prefer to carry one of my 336's in my woods to my 1895G loaded up with high-octane rounds. The 30-30 will do the job and accuracy and comfort is more important to me than the punch in this scenario. But again, back up in AK? I'd opt for my ported 1895G and I'd stoke it with Garret or Buffalo Bore (until I finally get my reloading going). I'd want hard cast with a wiiiide meplat and plenty of punch. Now, I'd opt for the 1895G because 1) I have one and 2) I like them. But that by no means makes them the best choice or the right choice for everyone. You'll find a whole lot more 12 gauges, .338's and .375's in AK than you will 45-70's.
I don't put much stock in magazine capacity and believe that's more hype and comfort than necessity. The odds of you even getting a shot off in such a scenario are slim to begin with and the odds of subsequent shots just diminish. But, I do like the idea of at least one or two more readily on board so a single shot wouldn't be my first choice. But, in the hands of a skilled user, a break-action single can be pretty darned fast and effective. Bolt actions can also be very fast.
My point being, you have to analyze your situations first. Then give very real and serious thought to what is truly best for you. Don't limit yourself, consider all the options out there from caliber to action to style and setup. Then shoot the ammo you're going to carry. Hard-hitting 45-70 is hard-hitting on both ends. You want to be prepared for that and be able to handle it and keep shots on target. A fella can walk around his living room with his tricked out 1895 feeling like Joe Badass but I guarantee you the bear will not be impressed. Take it serious and give it serious thought.
If you click the link in my signature for other links and then scroll down that thread you'll find a link to an older USFS paper on Alaska bear defense gun testing. It's interesting reading but it's very important to read all the test conditions, ammo, etc. The 45-70 didn't score well but they acknowledge they only had trapdoor-level factory 405's to test.
If you abuse the forum search function you should find a lot of discussions on bear defense from days past. Some of those old threads are little, shall we say lively and I might have been involved.
Again, I believe it is a serious subject worthy of serious consideration. I also think we should never be married to a gun/caliber to th epoint that we can't consider other options or have an open-enough mind to learn. Leastwise in big bear country. Where I'm at now I "worry" more about cats and dogs (cougars and wolves) than I do bears.
And we've tossed these vids around a few times before but I thought I'd toss them out there again. I don't agree with his carrying his Marlin unloaded but he's still pretty danged quick:
Single shot .375:
Marlin 45-70:
