[[Even a .22 rimfire can and has been used to kill the largest bears]]
A neighbor killed a bear with 22. Not advisable, but can be done. Poachers in our neck of the woods use 22’s on deer. Cam3 across 3 dead does, all shot with what looked like .22. The poachers musta got scared and fled, probably wqn5ing to go back at night and retrieve them.
The 10mm is becoming popular as a bear defense gun.
I think the old adage of picking the most powerful caliber you can shoot both accurately and quickly is good advice. Much depends on how much time you invest in practice.
The first few hours of practice, the gains are very great. Then the advances take longer and longer. At the highest level of competence, the shooters a practicing with hundreds to thousands of rounds a week.
You can reach a high level of competence with a hundred rounds a month and lots of dryfire or lazer simulation practice.
Practice drawing without firing will make a very significant difference in a short time.