Energy is horse****.
That big old bullet doesn't need ft-lb's like a little modern bullet. It just doesn't. A large diameter round or flat nose bullet doesn't burn up energy expanding when it hits the target, and the heavy bullet doesn't slow down as much inside the target.
Now I don't think a lever gun can shoot the 520 grain bullets I've used in a Sharps. But at 1000 fps or so, the 520 will go right through a buffalo, so you can't tell the entry from the exit hole when you're butchering it, turn the lungs to mush, and keep on going. That's a buffalo. A 405 should work fine on deer.
Hell, I saw it the 405 work just fine on
buffalo, not a deer, loaded with GOEX, not some hot modern load.
You might need 1000 ft-lb to drop a deer with a little .243, but not with a .458" 405 grain bullet. I think that the lower velocity actually has a more devastating effect. How much energy does a hunting arrow have? From what I've seen, a big .45-70 round almost works more like an arrow than a modern bullet. It kinda looks like an arrow, too.
What is difficult to master for a modern shooter who plinks with a .223 is the trajectory. If you can
hit the deer, whatever the range, you can kill it. That's the tough part, as the trajectory tables above demonstrate. I've also shot a jackrabbit with a black powder .45-70 Sharps at about 80 yards. Took a few rounds. I had the windage dead on, but I kept shooting over it, then under it, until I finally got the elevation right. That's how tough that trajectory can be, even relatively close in. When I finally connected, it went all the way through, surprisingly little damage to the rabbit, not at all like a modern varmint round. It blew the rabbit back about 8 feet, though.
Note that the old buffalo guns have marked ladder sights, which compensate for the trajectory much like an A2 sight on an AR, but they're a few inches tall, not a few 16ths tall like an A2 sight.
Also, the good shooters in the 1800s knew how to estimate range very precisely from the width of the front sight vs. the size of the animal, and they knew the trajectory of that bullet like we know our phone numbers today.