My favorite hunting unit in oregon is in the Eagle Cap wilderness. Elevation can vary from 3000-6700 feet. I still carry a 13 pound weapon. I just dont understand all the craze about ultralight weapons.
Where I grew up, and live, you can still be in the suburbs at 6,700 feet.
My lowest elevation hunts (antelope) have a base of 6,800 feet, even, going up to 6,920. For those hunts, I don't mind carrying a heavy rifle (10-13 lbs). It's pretty flat, and there's still plenty of oxygen.
But, the rest of my hunts generally have a base camp at 8,000 to 8,500 feet; and from there, all of the hunting is up and in rough terrain (exceeding 11,000 feet in a few places). Covering just 8-10 miles a day, at 9,000-10,500 feet, takes a lot more out of you, than most people expect.
At 8,000 feet, you have 17% less oxygen* in the atmosphere that at 3,000 feet. (26% less oxygen than at sea level.)
At 10,000 feet, you have 23% less oxygen than at 3,000 feet. (32% less oxygen than at sea level.)
The physical exertion required to navigate the terrain, coupled with the significantly lower oxygen levels, makes extra pounds feel like tons.
Even if you aren't winded, and struggling to breath, the lower oxygen levels still have quite an impact on your body.
Even standing still, your lungs have to work harder. Your heart has to work harder, because your O2 levels are down, but your body still needs just as much. But, no matter how hard they try, there will never be as much oxygen available, as at lower elevations. You are, in effect, in a constantly weakened state.
That 13 lb rifle that wasn't a problem at 6,000 feet now feels like it weighs 26 lbs at 9,500 feet, and
becomes a problem.
For the record... I'm healthy, under 160 lbs, and have been known to cover 20 miles in a day, while hunting. No ATVs. No trucks. ...Just one foot in front of the other.
*(The atmosphere contains 21% oxygen at all altitudes, but the lower atmospheric pressure at higher altitudes limits your ability to use that oxygen. So, an "effective oxygen level" is used to show the effect on the body. You can see a climbers' reference, here: Altitude Chart)