I'm a little late coming in on this but I think I have a perspective worth sharing. I have never been charged by a grizz, although they say there are grizzlies nearby, and although I've shot my share of blackies , I can't say that I've ever had a life threatening experience with them. Polar bears on the other hand, are a source of great interest to me. In our little community there are usually 3-6 defense kills per year, last year a large bear jumped on the hood of the CO's truck and they thought it would be in the cab with them before they could kill it. I've witnessed some shootings and participated in others. I've worked as a bear guard for contractors working in the area, and do wildlife control at the airport. One time a bear wandered out onto the apron right at flight time, lots of people on the apron. I went out there specifically to kill that bear thinking that was a safer course of action than attempting to scare it off. It didn't work out that way. The bear obviously read my mind, I circled around him putting the truck between him and the folks on the apron, at that point he bolted. He ran headlong into the chain link fence, peeling it off about 3 posts, then he tried to hide behind a crate. At that point I didn't have the heart to shoot him and was able to chase him off.
One year the DNR guys went into an area you've probably seen on polar bear specials on TV, where the bears interact with sled dogs. I guess they figured there were too many bears and too many stupid people and the situation had become unsafe, so they went in and darted everything in sight. There were more than a dozen bears there, and apparently they lost count of which bears they had darted. One young guy, a seasonal hire, got out of his truck, now I don't know what he was thinking, but he walks towards a darted bear without a gun!! Another bear to his, and fortunately to my left, was sleeping, not darted, gets up, and begins an intercept course with this young guy, who still hadn't even seen him. I'm thinking, well this one's for free, I idle my truck forward with the .416's snout out the window, wouldn't you know the damn bear turned off!
Over the last 23 years, I've personally had dozens of contacts which most would consider justifiable shooting situations, but I've held my fire and everyone went away unharmed. As a result I tend to work closer than I probably should; whether that bites me in the ass some day remains to be seen. But I've learned things like live fire is a poor scare device, and that the bear tends to slowly turn and walk away in a slow stiff legged, manner unless you hit him with rock fragments or pieces of frozen tundra from the bullet impact, then he'll bolt. And I've learned a bit about their personal space, and their reactions to having it infringed upon. Cracker shells combined with rubber bullets work best to move a bear along, and most bears will gallop off in an impressive manner from them. But I recall one female with cubs that got up against a hillside and even though I hammered her with rubber bullets, she wasn't going anywhere. I backed off when it appeared that she would charge. Although I wanted to push her out to the coast, it appeared there was some reason she didn't want to go that way. Perhaps there was a large male in the area that would kill her cubs. The upity learn-ned folks refer to small cubs as coy, an acronym for cubs of the year.
I have also discovered that as much as we might consider the big males as potentially the most dangerous, they are typically the bears you won't have trouble with. A female protecting her cubs are probably the most dangerous if you come upon them unexpectedly. Next would be a predatory bear which could be any bear of any size, ###, or age, best not to act like a seal, such as by sleeping in a dark shinny sleeping bag out on the sea ice while seal hunting and having lots of seal scent around. The next most dangerous are the teenagers. these guys are on their own for the first time, nothing scares them, they are always hungry, and way too curious to be safe. If you have trouble with a bear, chances are it will be one of these guys. The big males can be dangerous under the right conditions. My wife found out when she drove up beside one sleeping in a a big patch of seaweed. She was concentrating on several other bears out at the tide line, and she didn't see it until it became disturbed by her presence and got up. She has a pic of what looks like a chunk of limestone sticking out of the seaweed, which is a pretty common sight along the coast, and it appears right at the bottom edge of the truck window, she was actually taking a pic of the bears farther out. The next few pics are of one of the biggest bears I've ever seen, getting up and looking quite put out. All of these bears were feeding on a whale carcass, although the pic doesn't show it well, his coat is covered in blood.
Later she got this pic of him feeding.
Stumbling onto a bear unexpectedly at very close range remains the most likely way of getting into a shooting situation. Once that happens you are well inside their personal space, even though polar bears have a very small personal space compared to say a grizzly, which may act aggressively when you are still some distance away. Once inside their personal space, anything can happen, but what will happen is that they will approach you. That is unnerving to the uninitiated. When I've found myself in that situation, I've found that running directly at the bear puts him in a fight or flight dilemma, and fortunately for me, has thus far in my experience resulted in flight. A pal of mine tried it with a female with cubs,; he's luck to be alive. He managed to get back into the tour bus . . by the skin of her teeth. How can you stumble onto a white bear in open country unexpectedly? Well, sort of like this . . .