To move heavy loads on ones back, go with an external frame. I think that they use those alot up in the Boundry Waters area up north to portage and strap your loads to. Or do what the North Vietnamese did, strap it to a bicycle and push it! You could carry close to 500 lbs. from what I understand.
Thats a lot of rounds to carry on your back for self defense. What about food, water and shelter. I guess if you have enough ammo you can trade for (or take) that stuff. The later not recommended. You darn sure wont be the fastest mover on the block