Well, not to throw a wrench in anything but a great mountain rifle that is short, medium light, magazine fed and accurate is a ruger gunsite scout rifle. I bought this for a carry round in the truck, farm atv, multiple wolf, man defense rig. I had it accurized and installed a timney trigger and I got to tell you, for a shorter rig, it is extremely accurate with factory federal match sierra 168 grain boat tails. It shoots consistently 1/2 at 100 with just plain ole factory stuff. You have a choice of a 5 round or ten round magazine and it is a pleasure to carry. It's in 308 so one could load most anything---I just happen to have a buttload of federal match from a previous life so that's what I pack. I wouldn't hesitate to shoot it out to 600 ish as it holds moa to that distance. I carry it in a eberlestock mini me pack which works great for this kind of a rig. You also have a multitude of sighting options due to the fact that you can use the picitinney rail or the scope rings.
And like Broz, I've tried about every way to pack a rifle on a horse but I settled on pretty much a straight up carry. It is out of the way and you can pull the rifle out as you are dismounting for those really quick long range shots........