For a real back pack rifle, I would think in that connotation you are looking at a readily accessible rifle ready to use in a moments notice. What good is something packed away inside of the pack when you need it NOW. If you look over time, it has been the lever rifle of Marlin, Winchester and others that the cowboys and woodsman have had at their side for over a hundred years.
Most of these are quite rugged and lightweight and pack a punch at the same time. Another aspect is that you can take a .45 LC in rifle and revolver so one ammo type works in both. Likewise, .44 magnum and .357 lever rifles is another way to approach this issue.
For myself, I will soon get a Winchester .44 magnum rifle to go with my Ruger SRH and I will use the rifle as a camp gun and since it is so light, carry both when in the woods. I am one of those folks that shoots a rifle a hundred times better than a hand gun any day, but the Ruger SRH offers another powerful platform capable of taking even the large brown bears if needed. Once again, it is not the BEST for that job, but it has the power to do so nevertheless.
So, my personal choice is the .44 magnum rifle and revolver combination, and oh, by the way, I always carry my little SP101 .357 in pocket carry as may EDC and as a BUG when in the woods. Maybe a little over kill, but it works for me.