It is my understanding that small bores came about in the east as the larger game animals had been hunted so much that there wasn't much left except tons of small game for the cook pot around the settlements. This would possibly explain Crockett and his .40 cal rifle and the use of it on both black bear and small game.
Yes, history tells us that Crockett was an exceptional shot, but he wasn't the only exceptional shot to live in those days. He was just noted from his political career, and I have nothing but respect for the man.
Larger calibers came into their own once again when the the "woodsmen/ longhunters" crossed the Missouri and headed for the mountains. They soon found out that the small calibers were not going to down a grizzly along those streams where they were making their living trapping.
They needed the big bores once again and the Northwest Trade Gun was the deal to get. Yes, the Hawken brothers saw a demand for large caliber rifles and was making them for those that could afford them. Soon everyone was getting in on the action of gun making and the west is known for the big bore game getters, (kind of a rebirth of large bore muzzle loaders one might say) when folks started taking the lore of the grizzly attacks as fact.
Will a .40 take deer size game with round ball? Sure it will. It's the person behind the trigger that knows what they're doing that makes all the difference.
Is the .40 legal for deer size game where I live? Nope... While there are many folks still alive today with good hunting knowledge, the new group coming of age simply don't understand the round ball and its effectiveness with well placed shots. They don't bother to spend range time learning. Not all, but there is a large portion out there.
A good example is the person zeroing in on an 8 inch paper plate from 50 yards and their shots are all over it and they comment, "it's good enough for hunting..." With an attitude like that they could be using a .62 caliber for that matter, possibly even hit the deer,,, but if they didn't hit in the vitals they're no better off then the person who used the .40 caliber and zeroed in on a 3 inch dot and kept all the shots in it from 50 yards. I give the edge to the person using the .40 in this case that he/she will bring home some meat.