External frame will support the heaviest loads on moderate terrain and trails as the frame places most the weight directly on the hips. If the body does not have to get twisted by the terrain, these carry massive loads.
Internal frame will carry loads best on twisted terrain off trails as the frame distributes the weight on the hips and shoulders, and allows the body to flex somewhat while on the fly.
I have used both but primarily go with internal frame for pay and play with the Andinista my #1 used pack. For pure weight hauling of meat, 5gal fuel cans, firewood and such the Gregory Denali has no equal, the #1 used and seen pack in the mountains of Alaska. No pack carries heavy massive loads like the Gregory. I understand the rifle issue but it fits on my packs when I need my hands for ascending. Rifles for Alaskans who hunt for meat have badly bruised guns and I have seen broken and cracked stocks almost every year. For me, rifle stuffed inside a pack sleeve on me back does not have merit as I want my 338 in my hands just in case ole booboo pops out to get nasty, rarely happens but...
Look at Frontier Gear of Alaska packs.
I must also offer that buying a pack from the interweb and from recommendations from the interweb may not get you the perfect fitting pack. A pack should be bought in person from someone who knows how to fit packs. What fits my back and can be adjusted to fit better most likely can be ill fitting for you. But I know most of us want to ask, interweb buy and bounce down the trail with instant gratification Vs walk into a brick and mortar shop, get some questions thrown at you, have someone look you over and then recommend the pack to fit your body type then fine tune the pack to fit more better but then you may not be tactical savvy to your peers with that hang tag.
If you want THE premium pack look at McHale, no other pack made can match the detail and performance of McHale packs.
good luck
