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the reason you'd carry a small arm and a carbine of the same caliber was more for pragmatism's sake, though. It made no sense to carry around two different types of ammo, so that you never had a useless gun, and you never lost a fight because you were too busy fumbling around for the right stuff.
In an actual emergency scenario these days, that pragmatism becomes null, because it would be much harder to ride into town and find the local shopkeep. Likely, They won't just sell you precious ammo that they're using to defend their home.
There's an argument to carry arms for different calibers so you're more likely to scrounge something useful.