If I may, my understanding comes with no source but it makes the most sense to me. In the early days of exploring the frontiers of the new world, all firearms were muzzle loaded smooth bore muskets. The guns were long and of heavy construction. How much powder one used, and what one shot with that powder would be determined by what the target was. The load it would take to kill a deer at seventy yards would blow a rabbit, squirrel, or tweedy bird to smithereens at ten yards. I understand it wasn’t unusual to use rocks or salt as the projectile as they were about free compared to a lead ball. For even the lightest load you needed to use enough powder to get the projectile flying in a predictable straight line in order to aim. On the other side of the scale, a bear was the largest, most ‘bullet proof’ target. There are any number of firearms articles that will tell you that any modern handgun that isn’t a magnum will not even penetrate the skin of a bear. Bears are big, and tough, and hard as hell to knock down. Anyone who knows will tell you that shooting a bear with six .38 rounds will just piss him or her off and cause no permanent damage
So, back in the days when one long gun was used for all hunting and survival purposes, when bear were about, one loaded LOTS of powder and two balls, a dozen nails, and the kitchen sink. (Okay, you got me—there were no kitchens, or sinks back then.) Being loaded for bear, literally meant having your musket prepared with a load meant to bring down a bear. It should also be noted that it took a long, long time to reload. You got one shot, and it better bring him down and keep him down. Being loaded for bear was the extreme edge of preparing for an encounter with an animal that could completely eviscerate you with one swipe of a paw, and was way faster than you.
Eventually, firearms became better and cartridges were introduced. The phrase by that time had become adopted to mean “preparing for a worst case scenario battle”. It wasn’t about being armed; it was about being prepared to defend against an adversary that ordinary bullets literally bounced off of.
In reference to post #2, by the second half of the 1800’s, cartridges were pretty common. Since each cartridge had the same load as every other cartridge in that caliber, one went up in caliber to gain firepower. The most powerful rifle in common use at that time was most likely a ‘buffalo gun”. It seems buffalo are pretty hard to kill with one shot also. These guns were very limited however, a rifle that will drop a buffalo at a hundred yards, could not be used to hunt any small animals or even game. I mean a buffalo gun would surely kill a deer- but it might ruin a third to a half of the meat.
The older musket could be loaded with pellets for birds and rodents, maybe up to a turkey. A ball for game meat and possibly double powder loads for up to bear and moose hunting. Have to be pretty close though, not as accurate as a rifle.