Backpack with rifle holder

[Greenmist]

If 30L is too big, I'd consider a decent sling for the rifle and put some straps on a single army side pouch which is about 20L, another alternative would be a shoulder bag that hangs across the body (also go by the name bug out bag)

 

[easty3]

Definitely a vorn for me. Rifle sits right in the middle so balances perfectly