My backcountry partner has a Rem 700 in 300 RUM, long barrel, massive brake, Nighforce scope. The thing shoots like a .22, but hits way out beyond normal are an everyday thing with this rifle.
I, on the other hand, have a Tikka T3 Lite, stainless, .308 with an old Leupold Compact 2x7 VX-1. Shooting 165 Nosler Partitions at about 2750 and sighted 4" high at 100 yards. The rifle shoots 5/8" groups with ease.
I think the long range shot is possible if, as mentioned above, you have the right information and rifle. My partner carries a Leica rangefinder and the 'Shooter' app on his smart phone. We dialed in the info I could remember off the top of my head into the app, ranged a 2' square rock across a canyon at 786 yards then let the app calculate my drops. With no turrets, no mil-dots, just an estimated 10.8 foot hold over, my first shot was less than a foot high. Second shot, middle of the rock. I guess my point is, if you have the ability to range your target accurately and can establish your drops at the intended target range, you can likely hit at about any range within a few shots.
Would I take that shot at a muley? No way. But knowing that my rifle and chosen cartridge are up to the task and that my shooting technique is solid, I would take a shot at the muley 400 yards out. The hits at longer ranges just help build your confidence in your ability to hit at what your ethical limit is.
