IMO
Long guns and a BOB don't really go together, its a weight thing.
Get a short barrel for your 870 and carry 2 bx's of asst. ammo.
It still weights a bunch but well keep you safe like nothing else will. Remember the object is NOT to get into a situation that will require a weapon, you can't win. Lay down lead and get
away!
The post above to consider a Mini-14. While not a real military
weapon is a very good choice to fit your requirements.