So I've been doing some more thinking and reading on my second barrel. You can see some more details
here. A 44 Magnum looks like it could be a good backyard deer round. It would be about half the energy of my muzzleloader or a 45-70, but would still leave a big hole. Here are are some "generic" numbers that I found on the web. The 44 mag was using Remington ammo. I'm still excited for it to come in so I can go get it.

The next decisions will be which scopes to get.
Rifle Ballistics Summary 44 Magnum Ballistics Chart | Ballistics 101
44Magnum - Jacketed Hollow Point - 275 grains - 931 ft/lbs - 1,235fps
For comparison, here is a 45-70.
45-70 - Hollow Point - 300 grains - 2,182ft/lbs - 1,810fps