Ah, gentlemen, I've used my day pack several times to make 4-500 yd. shots. Often I have put it on a rock.
The trick is to nest into the pack in such a way as the gun is really steady. A bit of practice with your loaded day pack will make this a simple and easy thing to do.
I'd just add that taking long shots in the mountains is pretty tricky. That's because the air currents do funny things, particularly out past 400 yds. On deer class animals, that don't go far, even with a bad hit, might be one thing. But on the tougher stuff like elk ?
The other thing is the mean country. Wound something fairly tough and they will show you the worst of it before and if you find them. Guess how I learned that.
All the more reason why such shots should be given very serious consideration before attempted. E
