I've shot many deer with a .243 Nosler Partition, and although they expand and retain weight well, they don't usually get all that big. They're plenty sufficient for deer, but I would not shoot one at a bear. People often say "It's all about shot placement," but I think it's more accurate to say it's mostly about shot placement -- not quite all because every one of us has made a shot we'd like to have back.
For me, it comes down to this: In the case of marginal shots on bears, the .243 is a marginal cartridge. I'd compare it to a .222 for deer. Yes, a .222 has killed plenty of deer (including some by me). It's adequate when bullets are well placed, but the problem is that every bullet isn't well placed, and in those cases I want a bullet to dump lots of energy in a deer. I think we agree in essence because you said, "If it's the only rifle you got, I say go for it if you are a good enough shot." A lot depends on the phrase "if you are a good enough shot." My shooting skills are as good today as they were 30 years ago, but back then I had more confidence in my shooting than I do now. The difference is experience.
... in my area it would be tough to find comercially available 30-30 cartridges that I would recommend for bear hunting as they are primarily designed for deer hunters. i.e. Soft point.
Having said that many spring bear have very little body fat and therefore much of this conversation may not apply to your plans.
Good luck. I look forward to hearing about your success!
I'm taking a lever action Marlin clone .30-30 for sentimental reasons, mostly because it belonged to my brother who passed away a year and a half ago. If it weren't for that, I'd probably take either my bow, or my Savage 10ML muzzleloader. Last year a shot a doe with the .30-30, firing Federal Fusion bullets. It took her stem to stern, and I recovered the bullet just under the skin in her hind quarter after it broke the hip socket. That's pretty good penetration. I don't remember how much weight it retained, but it was somewhere around 80%, maybe a little more.