In my opinion, it seems to me, this thread is really covering 2 different debates. 1) semi-auto or revolver 2) how light of a cartridge can I use.
Debate #1
MUZZLE PRESS
The biggest difference is a revolver can be fired with the muzzle pressed into the attacker. Semi-autos can easily be made to do this too but the edge goes to revolvers.
AMMO SELECTION (variety)
Revolvers have less feeding issues with different ammo. Anyone going into the woods, combat, competition, etc with untested ammo (in any firearm) is an idiot. +1 revolver
RELIABILITY
Revolvers fail from dust and dirt and really are no different than semi-autos as a class in this regard. Some models of revolvers are NOT reliable when dirty just like some models of semi-autos aren't reliable when dirty. Revolvers get out of time, jam with dirt, freeze up when cold in wet conditions, and get bent from hot loads. Revolvers are not fool proof. It is up to the carrier to choose a model of either pistol or revolver that is reliable in dust, sand, water, mud and freezing tempts. There are many tests on this and of course if our lives depend upon these tools we will test this ourselves instead of trusting our holster queens and limit use of "expensive" ammo. Both ammo and firearm are disposable and should be treated as such. TIE
Debate #2
CALIBER OF CHOICE
All of us, regardless of the length of this thread, choose the smallest caliber we are comfortable shooting a bear with. Period.
DIAMETER
For those with semi autos, the 10 mm seems to be a very popular choice for this as an off-the-shelf option. For the wheel gun guys, the 44 magnum seems to be the smallest cartridge people are confident with, hence its popularity.
If larger diameter is what is important then why would anyone settle for a .44? If size really mattered, why not carry the largest? .600 Nitro seems like the ticket.
PENETRATION
If penetration is what is important to stop grizzly bears by reaching the brain or the spinal cord then penetration is what should be measured. Bullet selection drastically changes this measure for every cartridge as well as powder charge. A good 9mm is way better than a poor .44
CAPACITY
If capacity of bullets isn't important, why not carrie Thompson Contenders chambered in rifle cartridges? If capacity is important, then why aren't we all carrying KelTec PMR30 .22mag pistols with 30rnd
capacity?
At the end of the day, we each make a giant compromise on effectiveness with what we choose for bears protections.
I highly recommend anyone going into bear country to test their ammo. I highly recommend they throw their firearm in mud and sand and wet into a freezer to test the mechanism. And I highly recommend they practice applicable drills for bear country. Shoot "movers" Charging at the shooter from common brush distances. Shoot without sites. Include drawing from the holster WHILE wearing hunting gear. Don't be afraid to look like a fool wearing a bino harness and winter gloves at the gun range because I think we can all agree a bear is not going to wait for us to get ready.
Bear spray is an entirely different discussion full of misrepresented data and poor application that should be reserved for a different thread.