"You can't use 40 short and weak ammo in a 10mm"
This isn't true. Most 10mm handguns will function just fine running .40S&W. The case is the same except the 10mm is 3mm shorter. Also, most modern commercial 10mm ammo is downloaded to near factory .40S&W velocities. I don't know why you would carry a .40 for bear protection but for either the 10mm or the .40 in bear country you would want specialized deep penetration ammunition from someplace like Buffalo Bore or Underwood. Ammunition designed for defense against humans does not generally work well against heavy game as they often do not penetrate deep enough to reach vital organs and will fragment when they hit bone.

Here is an interesting read on handguns used in actual defensive encounters with bears.
https://www.ammoland.com/2024/05/bear-defense-with-handguns-update-20-more-cases-98-effective/
Go through the original and all the update links.

In many cases people stated that from the time they were aware of the bear to the bear being on them was around 2 seconds. Consider the type of holster and where you carry it as well. Most people cannot draw as quickly from a chest rig vs a holster on the hip.

My vote for a survival/bear gun would be a Remington 870 with a 14" barrel and ghost ring night sights. Add a sling and a receiver mounted ammo carrier. Carry different loads for different jobs. Birdshot for grouse/quail, maybe some buckshot and Brenneke Black Magic Magnum for bears. https://www.brennekeusa.com/brands/hunting-ammunition/black-magic-magnum

YMMV

topgun260 wrote:"You can't use 40 short and weak ammo in a 10mm"
This isn't true. Most 10mm handguns will function just fine running .40S&W. The case is the same except the 10mm is 3mm shorter. ...

I was curious about this since they both headspace on the case mouth.
From what I read, if the 40 will cycle in a 10mm pistol, it might work,
IF the extractor holds the case in the proper position.
If the extractor doesn't hold it, the cartridge will go farther forward until the case mouth bottoms out in the chamber--
then maybe the firing pin will still strike the primer (or maybe not).
Then maybe the empty case will eject, making follow-up shots possible (or maybe not).
Don't think I'd wanna bet my life on those maybes.

First of all, I wouldn't recommend using .40S&W ammo in a 10mm handgun for bear defense. If you have the 10mm handgun, use heavy game 10mm ammo in it designed for hunting or bear defense.

As for shooting .40 through a 10mm, yes, lots of people do it especially for practice as your budget .40 ammo is generally cheaper than 10mm.
I have personally shot hundreds of rounds of .40 through 10mm handguns with high reliability. There is plenty of info about this online if you search but in personal experience the Glock G20 and G40(long barrel 10mm) function fine using .40S&W. The Sig X-Ten runs fine on .40 too.

As always YMMV

topgun260 wrote:First of all, I wouldn't recommend using .40S&W ammo in a 10mm handgun for bear defense. If you have the 10mm handgun, use heavy game 10mm ammo in it designed for hunting or bear defense.

As for shooting .40 through a 10mm, yes, lots of people do it especially for practice as your budget .40 ammo is generally cheaper than 10mm.
I have personally shot hundreds of rounds of .40 through 10mm handguns with high reliability. There is plenty of info about this online if you search but in personal experience the Glock G20 and G40(long barrel 10mm) function fine using .40S&W. The Sig X-Ten runs fine on .40 too.

As always YMMV

Its not a 38 special in a 357, or 45 colt in a 454, which headspace on the rim. A rimless straightwall cartridge headspaces on the case mouth. A 45 auto is the same bullet diameter as a 45 colt, or 454, or 460, or 45 win mag, but you trying to shoot 45 acp in one of those is just dumb, buy the right ammo. When a rimmed cartridge goes into a chamber or cylinder, the rim locates the cartridge in the chamber. In a rimless bottleneck cartridge the cartridge is positioned by the shoulder. In a rimless straightwall, the cartridge is located in the chamber by the case mouth, which is why you never see a roll crimp on a rimless round. A 40 short and weak may fit in a 10mm chamber, however it is NOT the correct ammo. It will likely be pushed beyond the extractor by the firing pin, perhaps you can get rounds to fire with soft primers. But, the brass when it fires will set back to the bolt/slide, and maybe even cycle the slide, however, I really doubt that 40 ammo will feed from a magazine designed for rounds much longer. IF you have a very dirty chamber or have succeeded in firing enough 40 in a 10mm, there will likely be a carbon ring in front of the case mouth that will then very likely not permit a 10mm round to fit in the chamber.

There is a reason that 10mm guns do not have a placard saying 10mm or 40 S&W. Buy the correct ammo for the gun, not doing so is just dumb.