Well, to keep this firearms related, assuming you either have a holster in the bag or have a "tail" to carry a rifle. It depends on your budget and wants.
To keep with firearms, I love my Triple Aught Design Fast Pack Litespeed. It has a transport tail to carry a rifle and while it does the job, I wouldn't put it on terms of a suitable carrying case or for heavy mileage due to the rifle being so far off your back and shifting weight.
