For many years I shot 600 yard matches with NRA rules. We got five sighter shots before firing 20 shots for record. We shot from the prone position, with tight slings, with very accurate heavy rifles. The target was a 36" black circle on a white background. After each shot, people in the pits pulled the target down, stuck a spotter in it, and ran it back up, so you could see exactly where you had shot. We wore specialized shooting coats to dampen recoil, make our bodies rigid, and minimize heartbeat vibrations. Calibers ranged from the .223 using 80 grain bullets through .243s using 105 to 117 grain bullets, the 6.5mms, and .30 calibers. All bullets were heavy for caliber hollow point boat tails. Usually there were two scope sight matches and one iron sight match. The range, of course, was known exactly. Each of us had an exact zero from the previous match. We had three minutes to prepare and get into position. Our gear included a powerful spotting scope next to the rifle, in order to detect wind speed by reading the mirage. Most of us were pretty good at doing that. There were also large range flags to indicate wind speed and direction.
I maintain that the conditions of such matches were better for accuracy than one would ever find when hunting at comparable ranges.
Now here's the point. Why did we get five sighter shots? Because at least 1/3 of the first sighting shots did not come close enough to the target center to kill a deer. Also, it was quite common, after getting in the X-ring, to see a slight change in conditions blow the bullet out. In fact, anyone shooting 20 shots within a 12" circle would score 200/200 and probably win the match. Most of us couldn't do that.
From this experience, I think that hitting a deer at 600 yards with the first shot is larely a matter of luck. I would not attempt it. If the 6.5 is marginally better than a .270 beyond 600 yards, after it has dropped more in the first 600 yards, so what? Neither is a competent 600-yard hunting round.
You are largely correct about this, but there's two factors you're omitting: 1) Shooting off a bipod or ruck reduces group size by about a factor of two vs. prone+sling+coat 2) Wind is the primary enemy, and wind drift goes down as bore goes down and SD and BC goes up. Velocity matters too, but not as much. So there's a lot of loads in various .264 cartridges that can't be matched for wind drift by anything in a factory .277. A shooter with a 6.5-06 would have whipped all your .30-06 shooters quite handily. That's just the way the game works.
Nonetheless, between 500 and 550y is generally the practical limit to guarantee a first round hit in unknown wind in field conditions regardless of equipment. Sleek bullets and lots of powder only overcome the wind to a degree.
