Probably a shotgun, though SG ammo wieghs a great deal, it can handle all sorts of game. A good shotgunner's vest makes carrying easier.
When I venture out into the woods I'm usually carrying 24 rounds of pistol ammo and a dozen rounds of rifle ammo. Given this scenario, I'd leave the pistol ammo the same and up the rifle ammo to 100 rounds. I'd also want iron sights on my long gun (my current hunting rifle doesn't have those) in case my scope failed. As far as spare parts, if its by design you bet I'd have a spare firing pin and spring, as these usually go in my hunting pack anyway.
There are so few places left to explore for the average armchair commando, but there are plenty of people who have gone on year long walking tours (trans africa for instance) with no firearms. (not something I'd do)
As far as how much you can carry, I can assure you that for most 3-season north american backpacking 40 pounds is enough gear, 80 is asking for trouble. (unless that other 40 pounds is uncooked rice) Sure you'll end up washing the same clothes over and over, but if the kids on survivor can do it with a day pack for 40 days, you ought to be able to as well. As far as storing large game, making pemmican or biltong isn't that difficult, plan on packing a LOT of black pepper to keep the flies off your kill.
We always weighed our gear in ounces rather than pounds, you find very quickly that ounces add up.
You need some basic stuff, like a rainfly and groundcloth, though if you sleep in a hammock you can dispense with #2. You need a pot to cook in, you need a way to purify water. You need a way to carry water. You need raingear, because being wet for a long time can cause hypothermia. You need several changes of socks, cuz we all know dry socks are your best friend. You need some changes of clothes, and don't forget the ones you are already wearing. You need a sewing kit to repair the clothes you are wearing (nothing like losing the seat of your pants in the grand canyon to press this idea home). You need some spices because everything tastes better with a little pinch of home. You need batteries for things like flashlights and radios (or you could pack bay-gen free play radio with a light). You should use modern technology, because ther are things almost as good as wool and there are things better than cotton. Your sleeping bag should keep you warm even if its soaking wet, and thats why I'll never own a down bag. You need a toothbrush and a comb, because even Grizzly Adams had disdain for poor hygene. You need to learn to "tough it" without TP, and watch your diet... giardia can lead to dysentary in a hurry. And that will make you use all your TP at once. You need some tools, like a hatchet or shovel or cable saw because there are some jobs a knife can't do and digging a hole with your cooking pot is a lot tougher than it sounds. You need bug repellent.. (see giardia). You need some first aid stuff (asprin, gauze, etc) You need spare glasses, even if the prescription is old its better than no spares. You need matches, a lighter, calcium carbide and a metal match because fire is your friend and you need to be able to make it. You need a compass and a map or where you are going. You might need fishing line and wire for snares and , well fishing. You need to know whats edible and what's NOT where you are going (see giardia). You also need a positive attitude. Cuz being in the woods for a long time by yourself can make you a little crazy. You should take a friend if you can, makes hauling that rice easier.