Energy is not a reliable indicator of lethality when it comes to game
As far as a dangerous game stopping rifle, I have never heard of a single guide or professional hunter use a semi-auto
The reasons are simple, they are not 100% reliable and they are heavier and more unwieldy then other rifles
If you have hunted grizzlies then you know that the guides tell you to shoot just under the hump (side angle shot)
Why do they say this
Because they want the shot to break both shoulder bones and disable the bear
As far as the 45/70 goes, you can shoot traditional ammo that was loaded for trapdoor Springfields and the like
or
You can shoot factory loaded ammo designed for modern rifles like the guide gun, that puts the cartridge into a totally different category
or
If you have a Ruger #1 then you can really crank it up
Many Alaskan guides carry 45/70 guide guns as there stopping rifle, many others use bolt actions from 30/06 and up
Never heard of one packing a semi-auto
Never hear a guide or professional hunter advocate the spray and pray as a method for stopping dangerous game either